The Immigration and Nationality Act (INA) authorizesâand in some cases requiresâthe Department of Homeland Security (DHS) to detain non-U.S. nationals (aliens) arrested for immigration violations that render them removable from the United States. The immigration detention regime serves two primary purposes. First, detention may ensure an apprehended alien's presence at his or her removal hearing and, if the alien is ultimately ordered removed, makes it easier for removal to be quickly effectuated. Second, in some cases detention may serve the additional purpose of alleviating any threat posed by the alien to the safety of the community while the removal process is under way. The INA's detention framework, however, is multifaceted, with different rules turning on whether the alien is seeking initial admission into the United States or was lawfully admitted into the country; whether the alien has committed certain criminal offenses or other conduct rendering him or her a security risk; and whether the alien is being held pending removal proceedings or has been issued a final order of removal. In many cases detention is discretionary, and DHS may release an alien placed in formal removal proceedings on bond, on his or her own recognizance, or under an order of supervision pending the outcome of those proceedings. But in other instances, such as those involving aliens who have committed specified crimes, there are only limited circumstances when the alien may be released from custody. This report outlines the statutory and regulatory framework governing the detention of aliens, from an alien's initial arrest and placement in removal proceedings to the alien's removal from the United States. In particular, the report examines the key statutory provisions that specify when an alien may or must be detained by immigration authorities and the circumstances when an alien may be released from custody. The report also discusses the various legal challenges to DHS's detention power and some of the judicially imposed restrictions on that authority. Finally, the report examines how these legal developments may inform Congress as it considers legislation that may modify the immigration detention framework. The Supreme Court has long recognized that the federal government has "broad, undoubted power over the subject of immigration and the status of aliens," including with respect to their admission, exclusion, and removal from the United States. This authority includes the power to detain aliens pending determinations as to whether they should be removed from the country. The Court has predicated this broad immigration power on the government's inherent sovereign authority to control its borders and its relations with foreign nations. Notably, the Court has "repeatedly emphasized that 'over no conceivable subject is the legislative power of Congress more complete than it is over' the admission of aliens," and that "Congress may make rules as to aliens that would be unacceptable if applied to citizens." Despite the government's broad immigration power, the Supreme Court has repeatedly declared that aliens who have physically entered the United States come under the protective scope of the Due Process Clause of the Fifth Amendment, which applies "to all 'persons' within the United States, including aliens, whether their presence here is lawful, unlawful, temporary, or permanent." Due process protections generally include the right to a hearing and a meaningful opportunity to be heard before deprivation of a liberty interest. And one of the core protections of the Due Process Clause is the "[f]reedom from bodily restraint." But while the Supreme Court has recognized that due process considerations may constrain the federal government's exercise of its immigration power, there is some uncertainty regarding when these considerations may be consequential. Generally, aliens seeking initial entry into the United States typically have more limited constitutional protections than aliens present within the country. The Supreme Court has long held that aliens seeking entry into the United States have no constitutional rights regarding their applications for admission, and the government's detention authority in those situations seems least constrained by due process considerations. Thus, in Shaughnessy v. United States ex rel. Mezei , the Supreme Court upheld the indefinite detention of an alien who was denied admission into the United States following a trip abroad. The Court ruled that the alien's "temporary harborage" on Ellis Island pending the government's attempts to remove him did not constitute an "entry" into the United States, and that he could be "treated as if stopped at the border." Nevertheless, some courts have suggested that the constitutional limitations that apply to arriving aliens pertain only to their procedural rights regarding their applications for admission, but do not foreclose the availability of redress when fundamental liberty interests are implicated . Thus, some lower courts have concluded that arriving aliens have sufficient due process protections against unreasonably prolonged detention, and distinguished Mezei as a case involving the exclusion of an alien who potentially posed a danger to national security that warranted the alien's detention. Furthermore, regardless of the extent of their due process protections, detained arriving aliens may be entitled to at least some level of habeas corpus review, in which courts consider whether an individual is lawfully detained by the government. But due process considerations become more significant once an alien has physically entered the United States. As discussed above, the Supreme Court has long recognized that aliens who have entered the United States, even unlawfully, are "persons" under the Fifth Amendment's Due Process Clause. That said, the Court has also suggested that "the nature of that protection may vary depending upon [the alien's] status and circumstance." In various opinions, the Court has suggested that at least some of the constitutional protections to which an alien is entitled may turn upon whether the alien has been admitted into the United States or developed substantial ties to this country. Consequently, the government's authority to detain aliens who have entered the United States is not absolute. The Supreme Court, for instance, construed a statute authorizing the detention of aliens ordered removed to have implicit temporal limitations because construing it to allow the indefinite detention of aliens ordered removedâat least in the case of lawfully admitted aliens later ordered removedâwould raise "serious constitutional concerns." Declaring that the government's immigration power "is subject to important constitutional limitations," the Court has determined that the Due Process Clause limits the detention to "a period reasonably necessary to secure removal." Additionally, while the Supreme Court has recognized the government's authority to detain aliens p ending formal removal proceedings, the Court has not decided whether the extended detention of aliens during those proceedings could give rise to a violation of due process protections. But some lower courts have concluded that due process restricts the government's ability to indefinitely detain at least some categories of aliens pending determinations as to whether they should be removed from the United States. In sum, although the government has broad power over immigration, there are constitutional constraints on that power. These constraints may be most significant with regard to the detention of lawfully admitted aliens within the country, and least powerful with regard to aliens at the threshold of initial entry into the United States. From the outset, U.S. federal immigration laws have generally authorized the detention of aliens who are subject to removal. The first U.S. law on alien detention was the Alien Enemies Act in 1798, which subjected certain aliens from "hostile" nations during times of war to being detained and removed. But Congress passed no other laws on the detention of aliens for nearly a century. Starting in 1875, however, Congress enacted a series of laws restricting the entry of certain classes of aliens (e.g., those with criminal convictions), and requiring the detention of aliens who were excludable under those laws until they could be removed. In construing the government's detention authority, the Supreme Court in 1896 declared that "[w]e think it clear that detention or temporary confinement, as part of the means necessary to give effect to the provisions for the exclusion or expulsion of aliens, would be valid." Over the next few decades, Congress continued to enact laws generally mandating the detention and exclusion of proscribed categories of aliens seeking entry into the United States, as well as aliens physically present in the United States who became subject to removal. In 1952, Congress passed the INA, which distinguished between aliens physically arriving in the United States and those who had entered the country. Aliens arriving in the country who were found ineligible for entry were subject to "exclusion," and those already present in the United States who were found to be subject to expulsion were deemed "deportable." For aliens placed in exclusion proceedings, detention generally was required, unless immigration authorities, based on humanitarian concerns, granted the alien "parole," allowing the alien to enter and remain in the United States pending a determination on whether he or she should be admitted. In the case of deportable aliens, detention originally was authorized but not required, and aliens in such proceedings could be released on bond or "conditional parole." Congress later amended the INA to require, in deportation proceedings, the detention of aliens convicted of aggravated felonies, and authorized their release from custody only in limited circumstances, such as when the alien was a lawful permanent resident (LPR) who did not pose a threat to the community or a flight risk. In 1996, Congress enacted the Illegal Immigration Reform and Immigrant Responsibility Act (IIRIRA), which made sweeping changes to the federal immigration laws. IIRIRA replaced the INA's exclusion/deportation framework, which turned on whether an alien had physically entered the United States, with a new framework that turned on whether an alien had been lawfully admitted into the country by immigration authorities. Aliens who had not been admitted, including those who may have unlawfully entered the country, could be barred entry or removed from the country based on specified grounds of inadmissibility listed under INA Section 212. Aliens who had been lawfully admitted, however, could be removed if they fell under grounds of deportability specified under INA Section 237. A standard, "formal" removal proceeding was established for deportable aliens and most categories of inadmissible aliens. But IIRIRA created a new "expedited removal" process that applied to a subset of inadmissible aliens. This process applies to arriving aliens and certain aliens who recently entered the United States without inspection, when those aliens lack valid entry documents or attempted to procure their admission through fraud or misrepresentation. IIRIRA generally authorized (but did not require) immigration authorities to detain aliens believed to be removable pending those aliens' formal removal proceedings, but permitted their release on bond or "conditional parole." IIRIRA, however, required the detention of aliens who were inadmissible or deportable based on the commission of certain enumerated crimes or for terrorist-related grounds, generally with no possibility of release from custody. IIRIRA also generally required the detention of "applicants for admission," including aliens subject to expedited removal, pending determinations as to whether they should be removed (such aliens, however, could still be paroled into the United States by immigration officials in their discretion). This mandatory detention requirement has been applied even if those aliens were subsequently transferred to formal removal proceedings. Finally, IIRIRA created a detention scheme in which aliens with final orders of removal became subject to detention during a 90-day period pending their removal, and the government could (but was not required to) continue to detain some of those aliens after that period. A table showing the development of these immigration detention laws can be found in Table A-1 . Since IIRIRA's enactment, the statutory framework governing detention has largely remained constant. This detention framework is multifaceted, with different rules turning on whether the alien is seeking admission into the United States or was lawfully admitted within the country; whether the alien has committed certain enumerated criminal or terrorist acts; and whether the alien has been issued a final administrative order of removal. Four provisions largely govern the current immigration detention scheme: 1. INA Section 236(a) generally authorizes the detention of aliens pending formal removal proceedings and permits (but does not require) aliens who are not subject to mandatory detention to be released on bond or their own recognizance; 2. INA Section 236(c) generally requires the detention of aliens who are removable because of specified criminal activity or terrorist-related grounds; 3. INA Section 235(b) generally requires the detention of applicants for admission (e.g., aliens arriving at a designated port of entry) who appear subject to removal; and 4. INA Section 241(a) generally mandates the detention of aliens during a 90-day period after formal removal proceedings, and authorizes (but does not require) the continued detention of certain aliens after that period. While these statutes apply to distinct classes of aliens at different phases of the removal process, the statutory detention framework "is not static," and DHS's detention authority "shifts as the alien moves through different phases of administrative and judicial review." This section explores these detention statutes and their implementing regulations, including administrative and judicial rulings that inform their scope and application. (Other detention provisions in the INA that apply to small subsets of non-U.S. nationals, such as alien crewmen, or arriving aliens inadmissible for health-related reasons, are not addressed in this report. ) A table providing a comparison of these major INA detention statutes can be found in Table A-2 . INA Section 236(a) is the "default rule" for aliens placed in removal proceedings. The statute is primarily administered by Immigration and Customs Enforcement (ICE), the agency within DHS largely responsible for immigration enforcement in the interior of the United States. Section 236(a) authorizes immigration authorities to arrest and detain an alien pending his or her formal removal proceedings. Detention under INA Section 236(a) is discretionary, and immigration authorities are not required to detain an alien subject to removal unless the alien falls within one of the categories of aliens subject to mandatory detention (e.g., aliens convicted of specified crimes under INA Section 236(c), discussed later in this report). If ICE arrests and detains an alien under INA Section 236(a), and the alien is not otherwise subject to mandatory detention, the agency has two options: 1. it "may continue to detain the arrested alien" pending the removal proceedings; or 2. it "may release the alien" on bond in the amount of at least $1500, or on "conditional parole." Generally, upon release (whether on bond or conditional parole), the alien may not receive work authorization unless the alien is otherwise eligible (e.g., the alien is an LPR). And ICE may at any time revoke a bond or conditional parole and bring the alien back into custody. In the event of an alien's release, ICE may opt to enroll the alien in an Alternatives to Detention (ATD) program, which allows ICE the ability to monitor and supervise the released alien to ensure his or her eventual appearance at a removal proceeding. Following the arrest of an alien not subject to mandatory detention, an immigration officer may, at any time during formal removal proceedings, determine whether the alien should remain in custody or be released. But when an alien is arrested without a warrant, DHS regulations provide that the immigration officer must make a custody determination within 48 hours of the alien's arrest, unless there is "an emergency or other extraordinary circumstance" that requires "an additional reasonable period of time" to make the custody determination. DHS has defined "emergency or other extraordinary circumstance" to mean a "significant infrastructure or logistical disruption" (e.g., natural disaster, power outage, serious civil disturbance); an "influx of large numbers of detained aliens that overwhelms agency resources"; and other unique facts and circumstances "including, but not limited to, the need for medical care or a particularized compelling law enforcement need." After ICE's initial custody determination, an alien may, at any time during the removal proceedings, request review of that decision at a bond hearing before an immigration judge (IJ) within the Department of Justice's (DOJ's) Executive Office for Immigration Review. While the alien may request a bond hearing, INA Section 236(a) does not require a hearing to be provided at any particular time. If there is a bond hearing, regulations specify that it "shall be separate and apart from, and shall form no part of, any deportation or removal hearing or proceeding." During these bond proceedings, the IJ may, under INA Section 236(a), determine whether to keep the alien in custody or release the alien, and the IJ also has authority to set the bond amount. Following the IJ's custody decision, the alien may obtain a later bond redetermination only "upon a showing that the alien's circumstances have changed materially since the prior bond redetermination." Both the alien and DHS may appeal the IJ's custody or bond determination to the Board of Immigration Appeals (BIA), the highest administrative body charged with interpreting federal immigration laws. The filing of an appeal generally will not stay the IJ's decision or otherwise affect the ongoing removal proceedings. The BIA, however, may stay the IJ's custody determination on its own motion or when DHS appeals that decision and files a motion for a discretionary stay. Moreover, if ICE had determined that the alien should not be released or had set bond at $10,000 or greater, any order of the IJ authorizing release (on bond or otherwise) is automatically stayed upon DHS's filing of a notice of intent to appeal with the immigration court within one business day of the IJ's order, and the IJ's order will typically remain held in abeyance pending the BIA's decision on appeal. Following the enactment of IIRIRA, the DOJ promulgated regulations to govern discretionary detention and release decisions under INA Section 236(a). These regulations require the alien to "demonstrate to the satisfaction of the officer that . . . release would not pose a danger to property or persons, and that the alien is likely to appear for any future proceeding." Based on this regulation, the BIA has held that the alien has the burden of showing that he or she should be released from custody, and "[o]nly if an alien demonstrates that he does not pose a danger to the community should an [IJ] continue to a determination regarding the extent of flight risk posed by the alien." Some federal courts, however, have held that if an alien's detention under INA Section 236(a) becomes prolonged, a bond hearing must be held where the burden shifts to the government to prove that the alien's continued detention is warranted. For example, the U.S. Court of Appeals for the Ninth Circuit (Ninth Circuit ) has reasoned that, given an individual's "substantial liberty interest" in avoiding physical restraint, the government should prove by clear and convincing evidence that the detention is justified. The Supreme Court has not yet addressed the proper allocation of the burden of proof for custody determinations under INA Section 236(a). On the one hand, the Court has held that the statute does not itself require the government to prove that an alien's continued detention is warranted or to afford the alien a bond hearing. On the other hand, the Court has not decided whether due process considerations nonetheless compel the government to bear the burden of proving that the alien should remain in custody if detention becomes prolonged. While INA Section 236(a) and its implementing regulations provide standards for determining whether an alien should be released from ICE custody, they do not specify the factors that may be considered in weighing a detained alien's potential danger or flight risk. But the BIA has instructed that an IJ may consider, among other factors, these criteria in assessing an alien's custody status: whether the alien has a fixed address in the United States; the alien's length of residence in the United States; whether the alien has family ties in the United States; the alien's employment history; the alien's record of appearance in court; the alien's criminal record, including the extent, recency, and seriousness of the criminal offenses; the alien's history of immigration violations; any attempts by the alien to flee prosecution or otherwise escape from authorities; and the alien's manner of entry to the United States. The BIA and other authorities have generally applied these criteria in reviewing custody determinations. In considering an alien's danger to the community or flight risk, "any evidence in the record that is probative and specific can be considered." The BIA has also instructed that, in deciding whether an alien presents a danger to the community and should not be released from custody, an IJ should consider both direct and circumstantial evidence of dangerousness, including whether the facts and circumstances raise national security considerations. In addition, although bond proceedings are "separate and apart from" formal removal proceedings, evidence obtained during a removal hearing "may be considered during a custody hearing so long as it is made part of the bond record." Under DOJ regulations, an IJ may not determine the conditions of custody for classes of aliens subject to mandatory detention. In these circumstances, ICE retains exclusive authority over the alien's custody status. These limitations apply to arriving aliens in formal removal proceedings (including arriving aliens paroled into the United States); aliens in formal removal proceedings who are deportable on certain security and related grounds (e.g., violating espionage laws, criminal activity that "endangers public safety or national security," terrorist activities, severe violations of religious freedom); and aliens in formal removal proceedings who are subject to mandatory detention under INA Section 236(c) based on the commission of certain enumerated crimes. Although aliens who fall within these categories may not request a custody determination before an IJ, they may still seek a redetermination of custody conditions from ICE. In addition, aliens detained under INA Section 236(c) based on criminal or terrorist-related conduct may request a determination by an IJ that they do not properly fall within that designated category, and that they are thus entitled to a bond hearing. An alien may generally request review of ICE's custody determination at a bond hearing before an IJ, and the alien may also appeal the IJ's custody decision to the BIA. INA Section 236(e), however, expressly bars judicial review of a decision whether to detain or release an alien who is subject to removal: The Attorney General's discretionary judgment regarding the application of this section shall not be subject to review. No court may set aside any action or decision by the Attorney General under this section regarding the detention or release of any alien or the grant, revocation, or denial of bond or parole. Even so, the Supreme Court has determined that, absent clear congressional intent, INA provisions barring judicial review do not foreclose the availability of review in habeas corpus proceedings because "[i]n the immigration context, 'judicial review' and 'habeas corpus' have historically different meanings." Thus, despite INA Section 236(e)'s limitation on judicial review, the Court has held that the statute does not bar federal courts from reviewing, in habeas corpus proceedings, an alien's statutory or constitutional challenge to his detention. The Court has reasoned that an alien's challenge to "the statutory framework" permitting his detention is distinct from a challenge to the "discretionary judgment" or operational "decision" whether to detain the alien, which is foreclosed from judicial review under INA Section 236(e). Lower courts have similarly held that they retain jurisdiction to review habeas claims that raise constitutional or statutory challenges to detention. For that reason, although a detained alien may not seek judicial review of the government's discretionary decision whether to keep him or her detained, the alien may challenge the legal authority for that detention under the federal habeas statute. The Supreme Court has also considered whether a separate statute, INA Section 242(b)(9), bars judicial review of detention challenges. That statute provides: Judicial review of all questions of law and fact, including interpretation and application of constitutional and statutory provisions, arising from any action taken or proceeding brought to remove an alien from the United States under this subchapter shall be available only in judicial review of a final order [of removal] under this section. The Court has construed INA Section 242(b)(9) as barring review of three specific actions (except as part of the review of a final order of removal): (1) an order of removal, (2) the government's decision to seek removal (including the decision to detain the alien), and (3) the process by which an alien's removability would be determined. But the Court has declined to read the statute as barring all claims that could technically "arise from" one of those three actions. Thus, the Court has held that INA Section 242(b)(9) does not bar review of claims challenging the government's authority to detain aliens because such claims do not purport to challenge an order of removal, the government's decision to seek removal, or the process by which an alien's removability is determined. While INA Section 236(a) generally authorizes immigration officials to detain aliens pending their formal removal proceedings, INA Section 236(c) requires the detention of aliens who are subject to removal because of specified criminal or terrorist-related grounds. INA Section 236(c)(1) covers aliens who fall within one of four categories: 1. An alien who is inadmissible under INA Section 212(a)(2) based on the commission of certain enumerated crimes, including a crime involving moral turpitude, a controlled substance violation, a drug trafficking offense, a human trafficking offense, money laundering, and any two or more criminal offenses resulting in a conviction for which the total term of imprisonment is at least five years. 2. An alien who is deportable under INA Section 237(a)(2) based on the conviction of certain enumerated crimes, including an aggravated felony, two or more crimes involving moral turpitude not arising out of a single scheme of criminal misconduct, a controlled substance violation (other than a single offense involving possession of 30 grams or less of marijuana), and a firearm offense. 3. An alien who is deportable under INA Section 237(a)(2)(A)(i) based on the conviction of a crime involving moral turpitude (generally committed within five years of admission) for which the alien was sentenced to at least one year of imprisonment. 4. An alien who is inadmissible or deportable for engaging in terrorist activity, being a representative or member of a terrorist organization, being associated with a terrorist organization, or espousing or inciting terrorist activity. The statute instructs that ICE "shall take into custody any alien" who falls within one of these categories "when the alien is released [from criminal custody], without regard to whether the alien is released on parole, supervised release, or probation, and without regard to whether the alien may be arrested or imprisoned again for the same offense." While INA Section 236(c)(1) requires ICE to detain aliens who are removable on enumerated criminal or terrorist-related grounds, INA Section 236(c)(2) provides that ICE "may release an alien described in paragraph (1) only if" the alien's release "is necessary to provide protection to a witness, a potential witness, a person cooperating with an investigation into major criminal activity, or an immediate family member or close associate of a witness, potential witness, or person cooperating with such an investigation," and the alien shows that he or she "will not pose a danger to the safety of other persons or of property and is likely to appear for any scheduled proceeding." Under the statute, "[a] decision relating to such release shall take place in accordance with a procedure that considers the severity of the offense committed by the alien." Without these special circumstances, an alien detained under INA Section 236(c) generally must remain in custody pending his or her removal proceedings. Furthermore, given the mandatory nature of the detention, the alien may not be released on bond or conditional parole, or request a custody redetermination at a bond hearing before an IJ. Although an alien detained under INA Section 236(c) has no right to a bond hearing before an IJ, DOJ regulations allow the alien to seek an IJ's determination "that the alien is not properly included" within the category of aliens subject to mandatory detention under INA Section 236(c). The BIA has determined that, during this review, the IJ should conduct an independent assessment, rather than a "perfunctory review," of DHS's decision to charge the alien with one of the specified criminal or terrorist-related grounds of removability under INA Section 236(c). According to the BIA, the alien is not "properly included" within the scope of INA Section 236(c) if the IJ concludes that DHS "is substantially unlikely to establish at the merits hearing, or on appeal, the charge or charges that would otherwise subject the alien to mandatory detention." If the IJ determines that the alien is not properly included within INA Section 236(c), the IJ may then consider whether the alien is eligible for bond under INA Section 236(a). The mandatory detention requirements of INA Section 236(c) have been challenged as unconstitutional but, to date, none of these challenges have succeeded. In Demore v. Kim , an LPR (Kim) who had been detained under INA Section 236(c) for six months argued that his detention violated his right to due process because immigration authorities had made no determination that he was a danger to society or a flight risk. The Ninth Circuit upheld a federal district court's ruling that INA Section 236(c) was unconstitutional. The Ninth Circuit determined that INA Section 236(c) violated Kim's right to due process as an LPR because it afforded him no opportunity to seek bail. The Supreme Court reversed the Ninth Circuit's decision, holding that mandatory detention of certain aliens pending removal proceedings was "constitutionally permissible." The Court noted that it had previously "endorsed the proposition that Congress may make rules as to aliens that would be unacceptable if applied to citizens," and the Court also cited its "longstanding view that the Government may constitutionally detain deportable aliens during the limited period necessary for their removal proceedings, . . ." The Court concluded that "Congress, justifiably concerned that deportable criminal aliens who are not detained continue to engage in crime and fail to appear for their removal hearings in large numbers, may require that persons such as [Kim] be detained for the brief period necessary for their removal proceedings." The Court also distinguished its 2001 decision in Zadvydas v. Davis , where it declared that "serious constitutional concerns" would be raised if lawfully admitted aliens were indefinitely detained after removal proceedings against them had been completed. The Court reasoned that, unlike the post-order of removal detention statute at issue in Zadvydas , INA Section 236(c) "governs detention of deportable criminal aliens pending their removal proceedings ," and thus "serves the purpose of preventing deportable criminal aliens from fleeing prior to or during their removal proceedings, . . ." Yet in Zadvydas , removal was "no longer practically attainable" for the detained aliens following the completion of their proceedings, and so their continued detention "did not serve its purported immigration purpose." The Court further distinguished Zadvydas because that case involved a potentially indefinite period of detention, while detention under INA Section 236(c) typically lasts for a "much shorter duration" and has a "definite termination point"âthe end of the removal proceedings. Although the Supreme Court in Demore ruled that mandatory detention pending removal proceedings is not unconstitutional per se, the Court did not address whether there are any constitutional limits to the duration of such detention under INA Section 236(c). Some lower courts, however, have construed Demore to apply only to relatively brief periods of detention. Ultimately, in Jennings v. Rodriguez , the Supreme Court held that DHS has the statutory authority to indefinitely detain aliens pending their removal proceedings, but did not decide whether such prolonged detention is constitutionally permissible. INA Section 236(c)(1) instructs that ICE "shall take into custody any alien" who falls within one of the enumerated criminal or terrorist-related grounds " when the alien is released " from criminal custody. And under INA Section 236(c)(2), ICE may not release "an alien described in paragraph (1)" except for witness protection purposes. In its 2019 decision in Nielsen v. Preap , the Supreme Court held that INA Section 236(c)'s mandatory detention scheme covers any alien who has committed one of the enumerated criminal or terrorist-related offenses, no matter when the alien had been released from criminal incarceration. The Court observed that INA Section 236(c)(2)'s mandate against release applies to "an alien described in paragraph (1)" of that statute, and that INA Section 236(c)(1), in turn, describes aliens who have committed one of the enumerated crimes. The Court determined that, although INA Section 236(c)(1) instructs that such aliens be taken into custody "when the alien is released," the phrase "when . . . released" does not describe the alien, and "plays no role in identifying for the [DHS] Secretary which aliens she must immediately arrest." The Court thus held that the scope of aliens subject to mandatory detention under INA Section 236(c) "is fixed by the predicate offenses identified" in INA Section 236(c)(1), no matter when the alien was released from criminal custody. The Court also opined that, even if INA Section 236(c) requires an alien to be detained immediately upon release from criminal custody, ICE's failure to act promptly would not bar the agency from detaining the alien without bond. The Court relied, in part, on its 1990 decision in United States v. Montalvo-Murillo , which held that the failure to provide a criminal defendant a prompt bond hearing as required by federal statute did not mandate the defendant's release from criminal custody. Citing Montalvo-Murillo , the Court in Preap recognized the principle that if a statute fails to specify a penalty for the government's noncompliance with a statutory deadline, the courts will not "'impose their own coercive sanction.'" In short, the Court declared , "it is hard to believe that Congress made [ICE's] mandatory detention authority vanish at the stroke of midnight after an alien's release" from criminal custody. The Court thus reversed a Ninth Circuit decision that had restricted the application of INA Section 236(c) to aliens detained "promptly" upon their release from criminal custody, but noted that its ruling on the proper interpretation of INA Section 236(c) "does not foreclose as-applied challengesâthat is, constitutional challenges to applications of the statute as we have now read it." In sum, based on the Court's ruling in Preap , INA Section 236(c) authorizes ICE to detain covered aliens without bond pending their formal removal proceedings, regardless of whether they were taken into ICE custody immediately or long after their release from criminal incarceration. That said, the Court has left open the question of whether the mandatory detention of aliens long after their release from criminal custody is constitutionally permissible. The INA provides for the mandatory detention of aliens who are seeking initial entry into the United States, or who have entered the United States without inspection, and who are believed to be subject to removal. Under INA Section 235(b), an "applicant for admission," defined to include both an alien arriving at a designated port of entry and an alien present in the United States who has not been admitted, is generally detained pending a determination about whether the alien should be admitted into the United States. The statute thus covers aliens arriving at the U.S. border (or its functional equivalent), as well as aliens who had entered the United States without inspection, and are later apprehended within the country. The statute's mandatory detention scheme covers (1) applicants for admission who are subject to a streamlined removal process known as "expedited removal" and (2) applicants for admission who are not subject to expedited removal, and who are placed in formal removal proceedings. INA Section 235(b)(1) provides for the expedited removal of arriving aliens who are inadmissible under INA Section 212(a)(6)(C) or (a)(7) because they lack valid entry documents or have attempted to procure admission by fraud or misrepresentation. The statute also authorizes the Secretary of Homeland Security to expand the use of expedited removal to aliens present in the United States without being admitted or paroled if they have been in the country less than two years and are inadmissible on the same grounds. Based on this authority, DHS has employed expedited removal mainly to (1) arriving aliens; (2) aliens who arrived in the United States by sea within the last two years, who have not been admitted or paroled by immigration authorities; and (3) aliens found in the United States within 100 miles of the border within 14 days of entering the country, who have not been admitted or paroled by immigration authorities. More recently, however, DHS has expanded the use of expedited removal to aliens who have not been admitted or paroled, and who have been in the United States for less than two years (a legal challenge to this expansion is pending at the time of this report's publication). Generally, an alien subject to expedited removal may be removed without a hearing or further review unless the alien indicates an intention to apply for asylum or a fear of persecution if removed to a particular country. If the alien indicates an intention to apply for asylum or a fear of persecution, he or she will typically be referred to an asylum officer within DHS's U.S. Citizenship and Immigration Services (USCIS) to determine whether the alien has a "credible fear" of persecution or torture. If the alien establishes a credible fear, he or she will be placed in "formal" removal proceedings under INA Section 240, and may pursue asylum and related protections. INA Section 235(b)(1) and DHS regulations provide that an alien "shall be detained" pending a determination on whether the alien is subject to expedited removal, including during any credible fear determination; and if the alien is found not to have a credible fear of persecution or torture, the alien will remain detained until his or her removal. Typically, the alien will be initially detained by Customs and Border Protection (CBP) for no more than 72 hours for processing (e.g., fingerprints, photographs, initial screening), and the alien will then be transferred to ICE custody pending a credible fear determination if the alien is subject to expedited removal and requests asylum or expresses a fear of persecution. Under INA Section 212(d)(5), however, DHS may parole an applicant for admission (which includes an alien subject to expedited removal) on a case-by-case basis "for urgent humanitarian reasons or significant public benefit." Based on this authority, DHS has issued regulations that allow parole of an alien in expedited removal proceedings, but only when parole "is required to meet a medical emergency or is necessary for a legitimate law enforcement objective." INA Section 235(b)(1) provides that aliens who establish a credible fear of persecution or torture "shall be detained for further consideration of the application for asylum" in formal removal proceedings. The alien will typically remain in ICE custody during those proceedings. As noted above, DHS retains the authority to parole applicants for admission, and typically will interview the alien to determine his or her eligibility for parole within seven days after the credible fear finding. Under DHS regulations, the following categories of aliens may be eligible for parole, provided they do not present a security or flight risk: persons with serious medical conditions; women who have been medically certified as pregnant; juveniles (defined as individuals under the age of 18) who can be released to a relative or nonrelative sponsor; persons who will be witnesses in proceedings conducted by judicial, administrative, or legislative bodies in the United States; and persons "whose continued detention is not in the public interest." Under DHS regulations, a grant of parole ends upon the alien's departure from the United States, or, if the alien has not departed, at the expiration of the time for which parole was authorized. Parole may also be terminated upon accomplishment of the purpose for which parole was authorized or when DHS determines that "neither humanitarian reasons nor public benefit warrants the continued presence of the alien in the United States." For some time, the BIA took the view that aliens apprehended after unlawfully entering the United States (i.e., not apprehended at a port of entry), and who were first screened for expedited removal but then placed in formal removal proceedings following a positive credible fear determination, were not subject to mandatory detention under INA Section 235(b)(1). Instead, the BIA determined, these aliens could be released on bond under INA Section 236(a) because, unlike arriving aliens, they did not fall within the designated classes of aliens who are ineligible for bond hearings under DOJ regulations. Thus, the BIA concluded, INA Section 235(b)(1)'s mandatory detention scheme "applie[d] only to arriving aliens." In 2019, Attorney General (AG) William Barr overturned the BIA's decision and ruled that INA Section 235(b)(1)'s mandatory detention scheme applies to all aliens placed in formal removal proceedings after a positive credible fear determination, regardless of their manner of entry. The AG reasoned that INA Section 235(b)(1) plainly mandates that aliens first screened for expedited removal who establish a credible fear "shall be detained" until completion of their formal removal proceedings, and that the INA only authorizes their release on parole. The AG also relied on the Supreme Court's 2018 decision in Jennings v. Rodriguez , which construed INA Section 235(b) as mandating the detention of covered aliens unless they are paroled. Finally, the AG concluded, even though nonarriving aliens subject to expedited removal are not expressly barred from seeking bond under DOJ regulations, that regulatory framework "does not provide an exhaustive catalogue of the classes of aliens who are ineligible for bond." In a later class action lawsuit, the U.S. District Court for the Western District of Washington ruled that INA Section 235(b)(1)'s mandatory detention scheme is unconstitutional, and that aliens apprehended within the United States who are first screened for expedited removal and placed in formal removal proceedings following a positive credible fear determination are "constitutionally entitled to a bond hearing before a neutral decisionmaker" pending consideration of their asylum claims. The court thus ordered the government to (1) provide bond hearings within seven days of a bond hearing request by detained aliens who entered the United States without inspection, were first screened for expedited removal, and were placed in formal removal proceedings after a positive credible fear determination; (2) release any aliens within that class whose detention time exceeds that seven-day limit and who did not have a bond hearing; and (3) if a bond hearing is held, require DHS to prove that continued detention is warranted to retain custody of the alien. The DOJ has appealed the district court's ruling to the Ninth Circuit. The Ninth Circuit has stayed the lower court's injunction pending appeal insofar as it requires the government to hold bond hearings within seven days, to release aliens whose detention time exceeds that limit, and to require DHS to have the burden of proof. But the court declined to stay the lower court's order that aliens apprehended within the United States who are initially screened for expedited removal, and placed in formal removal proceedings after a positive credible fear determination, are "constitutionally entitled to a bond hearing." Thus, the Ninth Circuit's order "leaves the pre-existing framework in place" in which unlawful entrants transferred to formal removal proceedings after a positive credible fear determination were eligible for bond hearings. As a result of the district court's ruling, aliens apprehended within the United States who are initially screened for expedited removal and transferred to formal removal proceedings following a positive credible fear determination remain eligible to seek bond pending their formal removal proceedings. On the other hand, arriving aliens who are transferred to formal removal proceedings are not covered by the court's order, and generally must remain detained pending those proceedings, unless DHS grants parole. INA Section 235(b)(2) covers applicants for admission who are not subject to expedited removal. This provision would thus cover, for example, unadmitted aliens who are inadmissible on grounds other than those described in INA Section 212(a)(6)(C) and (a)(7) (e.g., because the alien is deemed likely to become a public charge, or the alien has committed specified crimes). The statute would also cover aliens who had entered the United States without inspection, but who are not subject to expedited removal because they were not apprehended within two years after their arrival in the country. The INA provides that aliens covered by INA Section 235(b)(2) "shall be detained" pending formal removal proceedings before an IJ. As discussed above, however, DHS may parole applicants for admission pending their removal proceedings, and agency regulations specify circumstances in which parole may be warranted (e.g., where detention "is not in the public interest"). Absent parole, aliens covered by INA Section 235(b)(2) generally must be detained and cannot seek their release on bond. INA Section 241(a) governs the detention of aliens after the completion of removal proceedings. The statute's detention authority covers two categories of aliens: (1) aliens with a final order of removal who are subject to detention during a 90-day "removal period" pending efforts to secure their removal; and (2) certain aliens who may (but are not required to) be detained beyond the 90-day removal period. The Supreme Court has construed the post-order of removal detention statute as having implicit temporal limitations. INA Section 241(a)(1) provides that DHS "shall remove" an alien ordered removed "within a period of 90 days," and refers to this 90-day period as the "removal period." The statute specifies that the removal period "begins on the latest of the following": The date the order of removal becomes administratively final. If the alien petitions for review of the order of removal, and a court orders a stay of removal, the date of the court's final order in the case. If the alien is detained or confined for nonimmigration purposes (e.g., criminal incarceration), the date the alien is released from that detention or confinement. INA Section 241(a)(2) instructs that DHS "shall detain" an alien during the 90-day removal period. The statute also instructs that "[u]nder no circumstance during the removal period" may DHS release an alien found inadmissible on criminal or terrorist-related grounds under INA Section 212(a)(2) or (a)(3)(B) (e.g., a crime involving moral turpitude); or who has been found deportable on criminal or terrorist-related grounds under INA Section 237(a)(2) or (a)(4)(B) (e.g., an aggravated felony conviction). The former Immigration and Naturalization Service (INS) previously issued guidance interpreting these provisions as only authorizing, but not requiring, the detention of "non-criminal aliens" during the 90-day removal period. There is no indication that DHS has rescinded that policy. But according to the agency, the statute generally requires the detention during the removal period of terrorists and aliens who have committed the specified crimes enumerated in the statute. Under this policy, however, if a criminal alien subject to mandatory detention has been granted withholding of removal or protection under the Convention Against Torture (CAT), the alien may be released if the agency is not pursuing the alien's removal. While INA Section 241(a)(1) specifies a 90-day removal period, it also provides that this period may be extended beyond 90 days and that the alien may remain in detention during this extended period "if the alien fails or refuses to make timely application in good faith for travel or other documents necessary to the alien's departure or conspires or acts to prevent the alien's removal subject to an order of removal." INA Section 241(a)(3) provides that, if the alien either "does not leave or is not removed within the removal period," the alien will be released and "subject to supervision" pending his or her removal. DHS regulations state that the order of supervision must specify the conditions of release, including requirements that the alien (1) periodically report to an immigration officer and provide relevant information under oath; (2) continue efforts to obtain a travel document and help DHS obtain the document; (3) report as directed for a mental or physical examination; (4) obtain advance approval of travel beyond previously specified times and distances; and (5) provide ICE with written notice of any change of address. Typically, an alien with a final order of removal is subject to detention during the 90-day removal period, and must be released under an order of supervision if the alien does not leave or is not removed within that period. INA Section 241(a)(6), however, states that an alien "may be detained beyond the removal period" if the alien falls within one of three categories: 1. an alien ordered removed who is inadmissible under INA Section 212(a) (e.g., an arriving alien who lacks valid entry documents); 2. an alien ordered removed who is deportable under INA Sections 237(a)(1)(C) (failure to maintain or comply with conditions of nonimmigrant status), 237(a)(2) (specified crimes including crimes involving moral turpitude, aggravated felonies, and controlled substance offenses), or 237(a)(4) (security and terrorist-related grounds); or 3. an alien whom DHS has determined "to be a risk to the community or unlikely to comply with the order of removal." DHS regulations provide that, before the end of the 90-day removal period, ICE will conduct a "custody review" for a detained alien who falls within one of the above categories, and whose removal "cannot be accomplished during the period, or is impracticable or contrary to the public interest," to determine whether further detention is warranted after the removal period ends. The regulations list factors that ICE should consider in deciding whether to continue detention, including the alien's disciplinary record, criminal record, mental health reports, evidence of rehabilitation, history of flight, prior immigration history, family ties in the United States, and any other information probative of the alien's danger to the community or flight risk. ICE may release the alien after the removal period ends if the agency concludes that travel documents for the alien are unavailable (or that removal "is otherwise not practicable or not in the public interest"); the alien is "a non-violent person" and likely will not endanger the community; the alien likely will not violate any conditions of release; and the alien does not pose a significant flight risk. Upon the alien's release, ICE may impose certain conditions, including (but not limited to) those specified for the release of aliens during the 90-day removal period, such as periodic reporting requirements. If ICE decides to maintain custody of the alien, it may retain custody authority for up to three months after the expiration of the 90-day removal period (i.e., up to 180 days after final order of removal). At the end of that three-month period, ICE may either release the alien if he or she has not been removed (in accordance with the factors and criteria for supervised release), or refer the alien to its Headquarters Post-Order Detention Unit (HQPDU) for further custody review. If the alien remains in custody after that review, the HQPDU must conduct another review within one year (i.e., 18 months after final order of removal), and (if the alien is still detained) annually thereafter. Although INA Section 241(a) authorizes (and in some cases requires) DHS to detain an alien after removal proceedings, the agency's post-order of removal detention authority has been subject to legal challenge, particularly when the alien remained detained indefinitely pending efforts to secure his or her removal to another country. Eventually, in Zadvydas v. Davis , a case involving the prolonged detention of lawfully admitted aliens who had been ordered removed, the Supreme Court interpreted the statute consistently with due process principles to limit detention generally to a six-month period after a final order of removal. In Zadvydas , the Supreme Court considered whether INA Section 241(a)'s post-order of removal detention statute should be construed as having an implicit time limitation to avoid serious constitutional concerns. The Court determined that "[a] statute permitting indefinite detention of an alien would raise a serious constitutional problem" under the Due Process Clause. The Court reasoned that "[f]reedom from imprisonmentâfrom government custody, detention, or other forms of physical restraintâlies at the heart of the liberty that Clause protects," and found no justifications for the indefinite detention of aliens whose removal is no longer practicable. While the Court recognized that a potentially indefinite detention scheme may be upheld if it is "limited to specially dangerous individuals and subject to strong procedural protections," INA Section 241(a)(6)'s post-removal period detention scheme was different because it applied "broadly to aliens ordered removed for many and various reasons, including tourist visa violations." The Court thus concluded that the statute could not be lawfully construed as authorizing indefinite detention. Notably, the Court rejected the government's contention that indefinite detention pending removal was constitutionally permissible under Shaughnessy v. United States ex rel. Mezei , which, many decades earlier, had upheld the indefinite detention on Ellis Island of an alien denied admission into the United States and ordered excluded. The Zadvydas Court distinguished Mezei , which involved an alien considered at the threshold of entry, because "once an alien enters the country, the legal circumstance changes, for the Due Process Clause applies to all 'persons' within the United States, including aliens, whether their presence here is lawful, unlawful, temporary, or permanent." The Zadvydas Court determined there was no indication that Congress had intended to confer immigration authorities with the power to indefinitely confine individuals ordered removed. Although INA Section 241(a)(6) states that an alien "may be detained" after the 90-day removal period, the Court reasoned, the statute's use of the word "may" is ambiguous and "does not necessarily suggest unlimited discretion." For these reasons, applying the doctrine of constitutional avoidance, the Court held that INA Section 241(a)(6) should be construed as authorizing detention only for "a period reasonably necessary to secure removal." The Court thus construed the statute as having an implicit temporal limitation of six months following a final order of removal. If that six-month period elapses, the Court held, the alien generally must be released from custody if he "provides good reason to believe that there is no significant likelihood of removal in the reasonably foreseeable future." In Clark v. Martinez , the Supreme Court considered whether the presumptive six-month time limitation established in Zadvydas applied to aliens who had not been lawfully admitted into the United States, and who were being detained after their 90-day removal periods had lapsed. The Court concluded that the time limitation read into INA Section 241(a)(6) for deportable aliens in Zadvydas equally applied to inadmissible aliens. But unlike in Zadvydas, the Court did not rest its decision on matters of constitutional avoidance. Instead, the majority opinion (written by Justice Scalia, who had dissented in Zadvydas ), relied on the principle of statutory construction that a provision should have the same meaning in different circumstances. "[B]ecause the statutory text provides for no distinction between admitted and nonadmitted aliens," the Martinez Court reasoned, the provision should be interpreted as having the same, presumptive six-month time limit for both categories of aliens. In reaching this conclusion, the Supreme Court rejected the government's invitation to construe the detention statute differently when applied to unadmitted aliens, which the government contended was proper because of the limited constitutional protections available to such aliens. The majority stated that "[b]e that as it may, it cannot justify giving the same detention provision a different meaning when such aliens are involved." Following the Supreme Court's decision in Zadvydas , the former INS issued regulations that established "special review procedures" for aliens who remain detained beyond the 90-day removal period. Under these rules, an alien may "at any time after a removal order becomes final" submit a written request for release because there is no significant likelihood of removal in the reasonably foreseeable future. The HQPDU will consider the alien's request and issue a decision on the likelihood of the alien's removal. Generally, if the HQPDU determines that there is no significant likelihood of removal, ICE will release the alien subject to any appropriate conditions. But if the HQPDU concludes that there is a significant likelihood of the alien's removal in the reasonably foreseeable future, the alien will remain detained pending removal. The regulations provide, however, that even if the HQPDU concludes that there is no significant likelihood of the alien's removal in the reasonably foreseeable future, the alien may remain detained if "special circumstances" are present. The regulations list four categories of aliens whose continued detention may be warranted because of special circumstances: (1) aliens with "a highly contagious disease that is a threat to public safety"; (2) aliens whose release "is likely to have serious adverse foreign policy consequences for the United States"; (3) aliens whose release "presents a significant threat to the national security or a significant risk of terrorism"; and (4) aliens whose release "would pose a special danger to the public." Some courts, though, have ruled that the former INS exceeded its authority by issuing regulations allowing the continued detention of aliens in "special circumstances." Both the Fifth and Ninth Circuits have concluded that the Supreme Court in Zadvydas never created an exception for the indefinite detention post-order of removal of aliens considered particularly dangerous. Instead, these courts concluded, the Supreme Court had merely suggested that it might be within Congress's power to enact a law allowing for the prolonged detention of certain types of aliens following an order of removal, not that Congress had done so when it enacted INA Section 241(a)(6), which does not limit its detention authority to "specific and narrowly defined groups." The Tenth Circuit, on the other hand, has ruled that the former INS's interpretation of the statute to permit indefinite detention in special circumstances was reasonable. The Supreme Court has not yet considered whether INA Section 241(a)(6) authorizes indefinite post-order of removal detention in special circumstances. As the above discussion reflects, DHS has broad authority to detain aliens who are subject to removal, and for certain classes of aliens (e.g., those with specified criminal convictions) detention is mandatory with no possibility of release except in limited circumstances. Further, while the Supreme Court has recognized limits to DHS's ability to detain aliens after removal proceedings, the Court has recognized that the governing INA provisions appear to allow the agency to detain aliens potentially indefinitely pending those proceedings. But some have argued that the prolonged detention of aliens during their removal proceedings without bond hearings is unconstitutional. Moreover, the government's ability to detain alien minors, including those accompanied by adults in family units, is currently limited by a binding settlement agreement known as theÂ  Flores Settlement, which generally requires the release of minors in immigration custody. Apart from concerns raised by prolonged detention, there has been criticism over the lack of regulations governing the conditions of confinement. Additionally, for aliens detained by criminal law enforcement authorities, DHS's authority to take custody of such aliens for immigration enforcement purposes through "immigration detainers" has been subject to legal challenge. The following sections provide more discussion of these developing issues. In Zadvydas v. Davis , discussed above, the Supreme Court in 2001 ruled that the indefinite detention of aliens after the completion of removal proceedings raised "a serious constitutional problem," at least for those who were lawfully admitted, and thus construed INA Section 241(a)(6)'s post-order of removal detention provision as containing an implicit six-month time limitation. In 2003, the Court in Demore v. Kim held that the mandatory detention of aliens pending removal proceedings under INA Section 236(c) was "constitutionally permissible," but did not decide whether there were any constitutional limits to the duration of such detention. Later, though, some lower courts ruled that the prolonged detention of aliens pending removal proceedings raised similar constitutional issues as those raised after a final order, and, citing Zadvydas , construed INA Section 236(c) as containing an implicit temporal limitation. In 2018, the Supreme Court held in Jennings v. Rodriguez that the government has the statutory authority to indefinitely detain aliens pending their removal proceedings, but left the constitutional questions unresolved. The Jennings case involved a class action by aliens within the Central District of California who had been detained under INA Sections 235(b), 236(c), and 236(a), in many cases for more than a year. The plaintiffs claimed that their prolonged detention without a bond hearing violated their due process rights. In 2015, the Ninth Circuit upheld a permanent injunction requiring DHS to provide aliens detained longer than six months under INA Sections 235(b), 236(c), and 236(a) with individualized bond hearings. The court expressed concern that the detention statutes, if construed to permit the indefinite detention of aliens pending removal proceedings, would raise "constitutional concerns" given the reasoning of the Supreme Court in Zadvydas . Although the Supreme Court in Demore had upheld DHS's authority to detain aliens without bond pending removal proceedings, the Ninth Circuit construed Demore's holding as limited to the constitutionality of "brief periods" of detention, rather than cases when the alien's detention lasts for extended periods. Recognizing the constitutional limits placed on the federal government's authority to detain individuals, the Ninth Circuit, as a matter of constitutional avoidance, ruled that the INA's detention statutes should be construed as containing implicit time limitations. The court therefore interpreted the mandatory detention provisions of INA Sections 235(b) and 236(c) to expire after six months' detention, after which the government's detention authority shifts to INA Section 236(a) and the alien must be given a bond hearing. The court also construed INA Section 236(a) as requiring bond hearings every six months. In addition, the court held that continued detention after an initial six-month period was permitted only if DHS proved by clear and convincing evidence that further detention was warranted. In Jennings , the Supreme Court rejected as "implausible" the Ninth Circuit's construction of the challenged detention statutes. The Court determined that the Ninth Circuit could not rely on the constitutional avoidance doctrine to justify its interpretation of the statutes. The Court distinguished Zadvydas , which the Ninth Circuit had relied on when invoking the constitutional avoidance doctrine, because the post-order of removal detention statute at issue in that case did not clearly provide that an alien's detention after the 90-day removal period was required. According to the Jennings Court, the statute at issue in Zadvydas was sufficiently open to differing interpretations that reliance on the constitutional avoidance doctrine was permissible. But the Jennings Court differentiated the ambiguity of that detention statute from INA Sections 235(b) and 236(c), which the Court held were textually clear in generally requiring the detention of covered aliens until the completion of removal proceedings. And the Court also observed that nothing in INA Section 236(a) required bond hearings after an alien was detained under that authority, or required the government to prove that the alien's continued detention was warranted after an initial six-month period. According to the Court, the Ninth Circuit could not construe the statutes to require bond hearings simply to avoid ruling on whether they passed constitutional muster. Having rejected the Ninth Circuit's interpretation of INA Sections 235(b), 236(a), and 236(c) as erroneous, the Court remanded the case to the lower court to address, in the first instance, the plaintiffs' constitutional claim that their indefinite detention under these provisions violated their due process rights. In short, the Jennings Court held that the government has the statutory authority to detain aliens potentially indefinitely pending their removal proceedings, but did not decide whether such indefinite detention is unc onstitutional . While the Supreme Court has not yet addressed the constitutionality of indefinite detention during removal proceedings, the Court had indicated in Demore v. Kim that aliens may be "detained for the brief period necessary for their removal proceedings." And in a concurring opinion in Demore , Justice Kennedy declared that a detained alien "could be entitled to an individualized determination as to his risk of flight and dangerousness if the continued detention became unreasonable or unjustified." After the Jennings decision, some lower courts have concluded that the detention of aliens during removal proceedings without a bond hearing violates due process if the detention is unreasonably prolonged. Some courts have applied these constitutional limitations to the detention of aliens arriving in the United States who are placed in removal proceedings, reasoning that, although such aliens typically have lesser constitutional protections than aliens within the United States, they have sufficient due process rights to challenge their prolonged detention. In reaching this conclusion, some courts have addressed the Supreme Court's 1953 decision in Shaughnessy v. United States ex rel. Mezei , which upheld the detention without bond of an alien seeking entry into the United States. These courts determined that Mezei is distinguishable because, in that case, the alien had already been ordered excluded when he challenged his detention, and the alien potentially posed a danger to national security that warranted his confinement. In addition, while the Jennings Court held that INA Section 236(a) does not mandate that a clear and convincing evidence burden be placed on the government in bond hearings, some courts have concluded that the Constitution requires placing the burden of proof on the government in those proceedings. At some point, whether in the Jennings litigation or another case, the Supreme Court may decide whether the indefinite detention of aliens pending removal proceedings is constitutionally permissible. In doing so, the Court may also reassess the scope of constitutional protections for arriving aliens seeking initial entry into the United States. The Court may also decide whether due process compels the government to prove that an alien's continued detention is justified at a bond hearing. The Court's resolution of these questions may clarify its view on the federal government's detention authority. As discussed, DHS has broad authority to detain aliens pending their removal proceedings, and in some cases detention is mandatory except in certain limited circumstances. But a 1997 court settlement agreement (the " Flores Settlement") currently limits the period in which an alien minor (i.e., under the age of 18) may be detained by DHS. Furthermore, under federal statute, an unaccompanied alien child (UAC) who is subject to removal is generally placed in the custody of the Department of Health and Human Services' Office of Refugee Resettlement (ORR), rather than DHS, pending his or her removal proceedings. In 2019, DHS promulgated a final rule that purports to incorporate these limitations with some modifications. The Flores Settlement originates from a 1985 class action lawsuit brought by a group of UACs apprehended at or near the border, who challenged the conditions of their detention and release. The parties later settled the plaintiffs' claims regarding the conditions of their detention, but the plaintiffs maintained a challenge to the INS's policy of allowing their release only to a parent, legal guardian, or adult relative. In 1993, following several lower court decisions, the Supreme Court in Reno v. Flores upheld the INS's release rule, reasoning that the plaintiffs had no constitutional right to be released to any available adult who could take legal custody, and that the INS's policy sufficiently advanced the government's interest in protecting the child's welfare. Ultimately, in 1997, the parties reached a settlement agreement that created a "general policy favoring release" of alien minors in INS custody. Under the Flores S ettlement, the government generally must transfer within five days a detained minor to the custody of a qualifying adult or a nonsecure state-licensed facility that provides residential, group, or foster care services for dependent children. But the alien's transfer may be delayed "in the event of an emergency or influx of minors into the United States," in which case the transfer must occur "as expeditiously as possible." In 2001, the parties stipulated that the Flores Settlement would terminate "45 days following [the INS's] publication of final regulations implementing this Agreement." In 2008, Congress enacted the William Wilberforce Trafficking Victims Protection Reauthorization Act of 2008 (TVPRA), which "partially codified the Flores Settlement by creating statutory standards for the treatment of unaccompanied minors." Under the TVPRA, a UAC must be placed in ORR's custody pending formal removal proceedings, and typically must be transferred to ORR within 72 hours after DHS determines that the child is a UAC. Following transfer to ORR, the agency generally must place the UAC "in the least restrictive setting that is in the best interest of the child," and may place the child with a sponsoring individual or entity who "is capable of providing for the child's physical and mental well-being." In 2015, the Flores plaintiffs moved to enforce the Flores Settlement, arguing that DHS (which had replaced the former INS in 2003) violated the settlement by adopting a no-release policy for Central American families and confining minors in secure, unlicensed family detention facilities. In response, the government argued that the Flores Settlement did not apply to accompanied minors. In an order granting the plaintiffs' motion, the federal district court ruled that the Flores Settlement applied to both accompanied and unaccompanied minors, and that accompanying parents generally had to be released with their children. In a later order, the court determined that, upon an "influx of minors into the United States," DHS may "reasonably exceed" the general five-day limitation on detention, and suggested that 20 days may be reasonable in some circumstances. In 2016, the Ninth Circuit upheld the district court's ruling that the Flores Settlement applies to both accompanied and unaccompanied minors, but held that the settlement does not require DHS to release parents along with their children. In any event, the effect of the Flores Settlement has been that DHS typically will release family units in their entirety pending removal proceedings, apparently because of the risks and difficulties that releasing the children only (while keeping the parents in detention) would pose, and the absence of a state licensing scheme for family detention facilities. Moreover, a federal district court has ruled that a "government practice of family separation without a determination that the parent was unfit or presented a danger to the child" likely violates due process. On August 23, 2019, DHS published a final rule that it claims "parallel[s] the relevant and substantive terms of the Flores Settlement" with some important modifications. Among other things, the rule c reates an alternative federal licensing scheme for DHS family detention facilities (which are not eligible for state licensing) that would enable DHS to detain minors together with their accompanying parents throughout the removal proceedings. This modification arguably conflicts with the Flores Settlement's " general policy favoring release" of alien minors from government custody. Yet DHS argues that the modification is compelled by changed circumstances, including the increased number of family unit apprehensions since 1997, and that detaining families together pending their removal proceedings "will enable DHS to maintain family unity " while enforcing federal immigration laws . Under the terms of the 2001 stipulation, the Flores Settlement will terminate 45 days after the government publishes final regulation s "implementing the A greement. " The key question in the Flores litigation likely will be whether the final rule " implement [s] the Agreement" within the meaning of the settlement's termination provision. If the court overseeing the Flores Settlement concludes that the rule meets that criteria, the DHS rule will effectively supersede the Flores Settlement. That said, w hile the final rule modifies the Flores Settlement to some degree, it largely incorporates the terms of that agreement . Thus, if the rule is upheld, DHS's detention authority over alien minors would remain subject to some constraints . Although the INA describes when an alien subject to removal may be detained and released from custody, neither the INA nor its implementing regulations currently provide any specific standards for the conditions of confinement. ICE, however, has developed "Performance-Based National Detention Standards" (PBNDS) governing the treatment of detained aliens. These standards apply to all ICE detention facilities, contract detention facilities, and state or local government facilities used by ICE through intergovernmental service agreements. The PBNDS require, among other things, clean and safe facilities; adequate food services; access to medical care; adequate bedding and personal hygiene; reasonable disability accommodations; communication and language assistance; access to telephone and mail; visitation rights; access to recreational programs; religious accommodations; work opportunities; and access to legal materials. In addition, CBP, the DHS component with primary responsibility for immigration enforcement along the border, has created similar standards governing the detention of aliens in CBP custody (e.g., arriving aliens in expedited removal proceedings). While the Supreme Court has generally addressed challenges to the duration of immigration detention, the Court has not addressed challenges to the conditions of immigration confinement. Lower courts, however, have considered detained aliens' constitutional challenges to the conditions of their confinement, generally under the standard applicable to pretrial detention in criminal cases. Under that standard, a detainee's conditions of confinement violate his or her right to due process if they amount to "punishment." To meet that threshold, a detainee must show that prison officials intended to punish him or her, or that the conditions of detention are not reasonably related to a legitimate governmental objective. More specifically, in cases involving claims of inadequate medical treatment, courts have typically analyzed such claims under the "deliberate indifference" standard. This standard looks to whether the detaining authority "knows of and disregards an excessive risk to inmate health or safety." In addition, even though aliens seeking initial entry into the United States typically have lesser constitutional protections than aliens within the United States, some courts have held that aliens detained at the border have substantive due process protections, such as the right to be free from "inhumane treatment" or "gross physical abuse." These cases suggest that aliens detained at the border may sometimes challenge the conditions of their confinement. In the past, some courts have rejected constitutional challenges to the conditions of immigration detention (or, in some cases, conditions of release), concluding that, while the alleged conditions may have been unpleasant or restrictive, they did not amount to a due process violation. As the Supreme Court once stated in a case about pretrial detention, "[l] oss of freedom of choice and privacy are inherent incidents of confinement in such a facility. And the fact that such detention interferes with the detainee's understandable desire to live as comfortably as possible and with as little restraint as possible during confinement does not convert the conditions or restrictions of detention into 'punishment.'" Other courts, however, have ruled unconstitutional conditions of immigration confinement that are particularly unreasonable, such as the deprivation of medical care and other basic necessities. As for minors, the Flores Settlement provides that those apprehended by DHS may be detained only in a "safe and sanitary" facility. The Flores Settlement also requires that state-licensed facilities comply with applicable state child welfare laws and building codes, and provide various services including routine medical care and education. In a few instances, the federal district court overseeing the Flores litigation has ruled that DHS violated the Flores Settlement by exposing minors to substandard conditions. Additionally, Congress, through appropriations legislation, has imposed certain requirements on the conditions of detention. For example, Congress has directed CBP and ICE to report their compliance with applicable detention facility standards (such as the PBNDS), and to provide certain other detention-related information, including the average length of detention and any instances in which an individual has died while in DHS custody. Thus, while federal statutes or regulations generally do not specify the standards for immigration detention, there are some important legal constraints on the treatment of detained aliens. Generally, upon issuing an administrative warrant, ICE may arrest and detain an alien pending a determination about whether the alien should be removed from the United States. But if an alien is in criminal custody by state or local law enforcement officers (LEOs) (e.g., if an alien is arrested by local police), ICE may take custody of the alien through the use of an "immigration detainer." An immigration detainer is a document by which ICE advises the LEOs of its interest in individual aliens whom the LEOs are detaining, and requests the LEOs to take certain actions that could facilitate removal (e.g., holding the alien temporarily, notifying ICE before releasing the alien). ICE's predecessor agency, the INS, had long issued detainers for potentially removable aliens in criminal custody. Eventually, in 1986, Congress enacted the Anti-Drug Abuse Act, which, among other things, explicitly authorized the use of detainers for deportable aliens who were arrested for violating controlled substance laws. Citing this authority, as well as its general immigration enforcement powers under the INA, the INS promulgated two separate regulations on detainers, one governing aliens arrested for controlled substance offenses, and another governing aliens arrested for other criminal offenses. In 1997, the INS merged both regulations into one, and that regulation is currently codified at 8 C.F.R. Â§ 287.7. The detainer regulation, as amended, provides the following: Any authorized immigration officer may at any time issue a Form I-247, Immigration Detainer-Notice of Action, to any other Federal, State, or local law enforcement agency. A detainer serves to advise another law enforcement agency that the Department seeks custody of an alien presently in the custody of that agency, for the purpose of arresting and removing the alien. The detainer is a request that such agency advise the Department, prior to release of the alien, in order for the Department to arrange to assume custody, in situations when gaining immediate physical custody is either impracticable or impossible. The regulation further instructs that, upon issuance of a detainer, the LEO "shall maintain custody of the alien for a period not to exceed 48 hours" beyond the time when the alien would have otherwise been released (excluding Saturdays, Sundays, and holidays) to facilitate transfer of custody to ICE. Although the detainer regulation instructs that LEOs "shall maintain custody" of an alien, reviewing courts have construed the regulation as being permissive rather than mandatory. For example, the Third Circuit has reasoned that the regulation calls a detainer a "request," that INA Section 287(d) does not require state or local LEOs to detain aliens subject to removal, and that DHS's (and the former INS's) policy statements have construed detainers as being "requests rather than mandatory orders." And the Third Circuit has also ruled that construing immigration detainers as mandatory would run afoul of the "anti-commandeering" principles of the Tenth Amendment, which prohibits the federal government from compelling state and local officials to enforce a federal regulatory scheme. As a result of judicial construction of the detainer regulation, LEOs may (but need not) notify ICE about an alien's release date and hold the alien pending transfer to ICE. Given the permissive nature of detainers, some state and local jurisdictions have restricted compliance with detainers except in limited circumstances (e.g., the alien has been convicted of or charged with a serious crime). Despite these restrictions, ICE generally issues detainers "[r]egardless of whether a federal, state, local, or tribal [LEO] regularly cooperates" with the detainer request. While DHS regulations authorize immigration detainers for removable aliens in criminal custody, courts have addressed legal challenges to the continued detention of aliens who would have otherwise been released from criminal custody (e.g., on bail, upon completion of sentence), but who remain detained pending their transfer to ICE. For example, in the past, ICE issued detainers so long as there was "reason to believe" the alien was subject to removal. But some courts have invalidated, on statutory or constitutional grounds, the use of detainers that are based only on ICE's representations about an alien's removability or the initiation of an investigation into the alien's immigration status. In Moreno v. Napolitano , a federal district court ruled that ICE's issuance of a detainer without an administrative arrest warrant exceeded its statutory authority under the INA absent a determination that the alien was likely to escape before a warrant could be obtained. In Morales v. Chadbourne , which involved the detention of a naturalized U.S. citizen, the First Circuit held that a detainer constitutes a new arrest under the Fourth Amendment, and must be supported by probable cause of the alien's removability. And in Orellana v. Nobles County , a federal district court held that a detainer claiming a "reason to believe" that an alien is subject to removal "does not provide a constitutionally sufficient basis" to detain an alien absent a "particularized assessment" of the alien's likelihood of escaping. In response to these court rulings, ICE in 2017 created new immigration detainer guidelines. Among other things, ICE officers "must establish probable cause to believe that the subject is an alien who is removable from the United States before issuing a detainer." And the detainer must come with either an administrative arrest warrant or a warrant of removal (if the alien has been ordered removed) signed by an authorized ICE officer. Despite ICE's revised detainer policy, some courts have held that, under the Fourth Amendment, immigration detainers supported by probable cause that an alien is removable still do not justify the alien's continued detention by state or local LEOs unless there is probable cause that the alien has committed a criminal offense giving those LEOs a basis to detain the alien for criminal prosecution. These rulings are largely informed by the Supreme Court's 2012 decision in Arizona v. United States , which held that a state statute authorizing police officers unilaterally to arrest an alien suspected of being removable was preempted by federal law, which exclusively gave the authority to enforce civil immigration laws to federal immigration officers. So these courts reason, because state and local LEOs generally lack the authority to enforce civil immigration laws, they may not hold an alien under an immigration detainer unless there is an independent basisâsuch as probable cause of a crimeâto justify the continued detention. In City of El Cenizo v. Texas , however, the Fifth Circuit held that state and local LEOs do not need probable cause of a crime to hold an alien pursuant to an immigration detainer. The court reasoned that many state laws permit seizures without probable cause of a crime, such as those relating to mentally ill individuals, and that "civil removal proceedings necessarily contemplate detention absent proof of criminality." The circuit court also distinguished Arizona because that case "involved unilateral status-determinations [by the state] absent federal direction ," while a detainer "always requires a predicate federal request before local officers may detain aliens for the additional 48 hours." Courts are thus divided over whether immigration detainers are permissible under the Fourth Amendment. Some courts have held that a detainer need be supported only by probable cause of an alien's removability to avoid constitutional violations, while other courts require probable cause of criminal activity before an alien may be held pending transfer to ICE. Given that ICE considers detainers to be integral to its efforts to arrest and remove aliens convicted of specified crimes, the split in court opinion on the circumstances when detainers may be honored could have significant consequences for ICE's enforcement policies in different jurisdictions. DHS generally has substantial authority to detain aliens who are subject to removal. But the governing laws on detention may differ depending on the circumstances, including (1) whether the alien is seeking initial admission into the United States or had been lawfully admitted into the country; (2) the type of removal proceedings in which the alien is placed; (3) whether the alien has committed specified criminal or terrorist-related activity; (4) whether the alien is a UAC or falls within some other category subject to special rules for detention; and (5) whether the alien is being held for formal removal proceedings or has been ordered removed and is awaiting effectuation of the removal order. Typically, DHS may detain aliens who are placed in formal removal proceedings, but may release the alien on bond, on his or her own recognizance, or under an order of supervision pending the outcome of those proceedings. In some cases, such as those involving aliens who have committed specified crimes, or aliens arriving in the United States who are placed in expedited removal proceedings, detention is mandatory and the alien may not be released from custody except in limited circumstances. Furthermore, DHS generally must detain aliens who have received final orders of removal for up to 90 days while their removal is effectuated, and the agency retains the discretion to detain certain classes of aliens after that 90-day period has lapsed. However, there are some constraints on DHS's detention power. The Supreme Court has determined that the indefinite detention of aliens after formal removal proceedings would raise "serious constitutional concerns," at least for those who were lawfully admitted into the United States and became subject to removal. And while the Court has recognized that governing statutes confer broad authority to DHS to detain aliens without bond pending their removal proceedings, some lower courts have held that due process requires the government to provide detained aliens with bond hearings after prolonged periods of detention and to prove that any continued detention is justified. Furthermore, DHS's ability to detain family units pending their proceedings remains constrained by the Flores Settlement, which limits the length of detention of alien minors. In addition, while detention litigation has largely centered on the duration of detention, detained aliens have also sometimes brought challenges to the conditions of their confinement. And more recently, some courts have imposed restrictions on DHS's ability to take custody of aliens in state or local law enforcement custody through immigration detainers. As courts continue to grapple over the scope of DHS's detention power, Congress may consider legislative proposals that would either limit or expand that authority. For instance, some recent bills would end mandatory detention entirely, afford all aliens the opportunity to be released on bond pending removal proceedings, and require DHS to prove that any continued detention is warranted. Certain bills would also require DHS to promulgate regulations for detention facilities; require the periodic inspection of those facilities; or impose standards governing the conditions of detention, such as requiring medical screenings and access to food, water, shelter, and hygiene. As for custody determinations, some bills would require DHS to consider ATD programs instead of bond or conditional parole, and require placing some aliens in such programs (e.g., asylum applicants). Other bills would generally require the release of aliens considered "vulnerable," such as those who are detained with children, and limit the amount of any bond. In addition, some bills would create time limitations for an IJ to conduct bond hearings, and require periodic bond hearings while an alien remains in custody. Conversely, some bills would specify that an alien may be detained for an indefinite period pending removal proceedings, and require the alien to prove by clear and convincing evidence that he or she is not a flight or escape risk in order to be released. Some bills would also expand the classes of aliens subject to mandatory detention to include aliens present in the United States without inspection, criminal gang members, and aliens arrested for (but not yet convicted of) specified crimes. Other bills would override the Flores Settlement effectively to extend INA Section 235(b)(1)'s mandatory detention scheme governing applicants for admission to family units. Finally, some bills would clarify DHS's detainer authority to provide that ICE may issue detainers so long as there is probable cause that an alien is removable. In short, as reviewing courts continue to test the outer limits to DHS's detention authority, Congress may consider additional legislative options that inform the scope of that authority. The following tables provide (1) an overview of the development of U.S. immigration detention laws, and (2) a comparison of the various detention regimes under current law.

Summary: The Immigration and Nationality Act (INA) authorizesâand in some cases requiresâthe Department of Homeland Security (DHS) to detain non-U.S. nationals (aliens) arrested for immigration violations that render them removable from the United States. An alien may be subject to detention pending an administrative determination as to whether the alien should be removed, and, if subject to a final order of removal, pending efforts to secure the alien's removal from the United States. The immigration detention scheme is multifaceted, with different rules that turn on several factors, such as whether the alien is seeking admission into the United States or has been lawfully admitted into the country; whether the alien has engaged in certain proscribed conduct; and whether the alien has been issued a final order of removal. In many instances DHS maintains discretion to release an alien from custody. But in some instances, such as when an alien has committed specified crimes, the governing statutes have been understood to allow release from detention only in limited circumstances. The immigration detention scheme is mainly governed by four INA provisions that specify when an alien may be detained: 1. INA Section 236(a) generally authorizes the detention of aliens pending removal proceedings and permits aliens who are not subject to mandatory detention to be released on bond or on their own recognizance; 2. INA Section 236(c) generally requires the detention of aliens who are removable because of specified criminal activity or terrorist-related grounds after release from criminal incarceration; 3. INA Section 235(b) generally requires the detention of applicants for admission, such as aliens arriving at a designated port of entry as well as certain other aliens who have not been admitted or paroled into the United States, who appear subject to removal; and 4. INA Section 241 (a) generally requires the detention of aliens during a 90-day period after the completion of removal proceedings and permits (but does not require) the detention of certain aliens after that period. These provisions confer substantial authority upon DHS to detain removable aliens, but that authority has been subject to legal challenge, particularly in cases involving the prolonged detention of aliens without bond. DHS's detention authority is not unfettered, and due process considerations may inform the duration and conditions of aliens' detention. In 2001, the Supreme Court in Zadvydas v. Davis construed the statute governing the detention of aliens following an order of removal as having implicit, temporal limitations. The Court reasoned that construing the statute to permit the indefinite detention of lawfully admitted aliens after their removal proceedings would raise "serious constitutional concerns." In 2003, however, the Court in Demore v. Kim ruled that the mandatory detention of certain aliens pending their removal proceedings, at least for relatively brief periods, was constitutionally permissible. The interplay between the Zadvydas and Demore rulings has called into question whether the constitutional standards for detention prior to a final order of removal differ from those governing detention after a final order is issued. Several lower courts have interpreted Demore to mean that mandatory detention pending removal proceedings is not per se unconstitutional, but that Zadvydas cautions that if this detention becomes "prolonged" it may not comport with due process requirements. Additionally, some lower courts have recognized constraints on DHS's detention power that the Supreme Court has not yet considered. For instance, some courts have ruled that the Due Process Clause requires aliens in removal proceedings to have bond hearings when detention becomes prolonged, where the government bears the burden of proving that the alien's continued detention is justified. In addition, a settlement agreement known as the " Flores Settlement," which is enforced by a federal district court, currently limits DHS's ability to detain alien minors who are subject to removal. Further, while litigation concerning immigration detention has largely centered on the duration of detention, some courts have considered challenges to the conditions of immigration confinement, generally under the standards applicable to pretrial detention in criminal cases. Some courts have also restricted DHS's ability to take custody of aliens detained by state or local law enforcement officials upon issuance of "immigration detainers." In short, while DHS generally has broad authority over the detention of aliens, that authority is not without limitation. As courts continue to grapple with legal and constitutional challenges to immigration detention, Congress may consider legislative options that clarify the scope of the federal government's detention authority.