Congress might consider reforms to the nation's immigration system, and, thus, the detention of noncitizens (aliens) in the United States may be an issue. Congressional interest in the policy of detaining noncitizens in the United States while determining whether noncitizens should be removed from the United States tends to be varied. For example, while some want to increase the categories of aliens who are detained and increase the amount of detention space, others want to create alternatives to detention and exempt asylum seekers from mandatory detention. In addition, immigration enforcement activities affect the need for and allocation of detention resources. For example, as the Department of Homeland Security (DHS) expands programs to locate removable aliens from jails throughout the country, DHS may need additional detention beds in areas of the United States where traditionally there has not been a need for detention space. The Immigration and Nationality Act (INA) provides broad authority to detain aliens while awaiting a determination of whether they should be removed from the United States and mandates that certain categories of aliens are subject to mandatory detention (i.e., the aliens must be detained) by DHS. Aliens not subject to mandatory detention may be detained, paroled, or released on bond. Any alien can be detained while DHS determines whether the alien should be removed from the United States. Approximately half of the detained aliens have committed a crime while in the United States, have served their criminal sentence, and are detained while undergoing deportation proceedings. Other detained aliens include those who are arrested in the United States for being illegally present, or who arrived at a port-of-entry without proper documentation (e.g., fraudulent or invalid visas, or no documentation), but most of these aliens are quickly returned to their country of origin through a process known as expedited removal. The majority of aliens arriving without proper documentation who claim asylum are held until their "credible fear hearing," but some asylum seekers are held until their asylum claims have been adjudicated. This report provides an overview of immigration detention in the United States. It begins with a discussion of the authority to detain noncitizens while awaiting a determination of whether the noncitizen is removable from the United States. The report continues with a discussion of the resources for immigration detention and concludes with a discussion of legislation in the 112 th Congress related to detention. The report also presents general policy issues surrounding detention of aliens, including concerns about the number of aliens subject to mandatory detention; the justness of mandatory detention, especially as it is applied to asylum seekers arriving without proper documentation; and the proper limits on the length of time in detention for aliens who have been ordered removed but cannot be returned to their home country. The daily responsibility for detaining aliens resides with the Secretary of Homeland Security, but under law the Attorney General may still retain the authority to arrest and detain aliens. The INA provides the Attorney General with broad authority to detain aliens while awaiting a determination of whether they should be removed from the United States. The creation of DHS moved the administration of detention of noncitizens from the Department of Justice's Immigration and Naturalization Service (INS) to DHS. While current regulations vest all authorities and functions of the DHS to administer and enforce the immigration laws with the Secretary of Homeland Security (hereafter the Secretary) or his delegate, it can be argued that the language in the Homeland Security Act of 2002 (HSA) has left the Attorney General with concurrent authority over some aspects of immigration law. In addition, both the Department of Justice, through the Executive Office of Immigration Review (EOIR), and DHS have authority for determining bond for aliens. Officials within DHS also make bond determinations that may or may not subsequently come before EOIR. The Board of Immigration Appeals (BIA), the appellate body within EOIR, hears appeals from matters decided by immigration judges. The BIA has jurisdiction to consider appeals of various decisions made by immigration officials in DHS, including the granting of bond. The Attorney General has final say in matters of immigration law that come before EOIR. For example, on April 17, 2003, the Attorney General released a decision that instructs immigration judges to consider "national security interests implicated by the encouragement of further unlawful mass migrations" in making bond determinations for unauthorized migrants who arrive in "the United States by sea seeking to evade inspection." In the decision, the Attorney General states that he retains the authority to detain or authorize bond for aliens, but the authority is "shared" with the Secretary since DHS's officials make the initial determination whether an alien will remain in custody during removal proceedings. As discussed above, the INA gives the Attorney General the authority to issue a warrant to arrest and detain any alien in the United States while awaiting a determination of whether the alien should be removed from the United States. The Illegal Immigrant Reform and Immigrant Responsibility Act of 1996 (IIRIRA) amended the INA, effectively specifying levels of detention priority and classes of aliens subjected to mandatory detention. Mandatory detention is required for certain criminal and terrorist aliens who are removable, pending a final decision on whether the alien is to be removed. No bail is available and only a hearing can determine whether the alien qualifies as a criminal or terrorist alien. Aliens not subjected to mandatory detention can be paroled, released on bond, or continue to be detained. Decisions on parole made by the Secretary and bond decisions made by the Attorney General are not subject to review. In October 1998, the former INS issued a memorandum, which is still in effect, establishing detention guidelines consistent with the changes made by IIRIRA. According to the guidelines, detainees are assigned to one of four detention categories: (1) required; (2) high priority; (3) medium priority; and (4) lower priority. Aliens in required detention must be detained (e.g., criminal aliens, national security risks) while aliens in the other categories may be detained depending on detention space and the facts of the case. Higher priority aliens should be detained before aliens of lower priority. Additionally, INA Section 236A requires the detention of an alien whom the Attorney General certifies as someone who the Attorney General has "reasonable grounds" to believe is involved in terrorist activities or in any other activity that endangers national security. The Attorney General must initiate removal proceedings or bring criminal charges within seven days of arresting the alien or release the alien. An alien who is detained solely as a certified terrorist, who has not been removed, and who is unlikely to be removed in the foreseeable future may be detained for periods of up to six months only if his release would pose a danger to national security or public safety. The Attorney General must review the terrorist certification every six months. Under the INA, the Attorney General also has the authority to arrest and detain aliens without a warrant if he has "reason to believe that the alien ... is in the United States in violation of any [immigration] law and is likely to escape before a warrant can be obtained." Functionally, DHS is responsible for arresting and detaining aliens. If an alien is arrested without a warrant, a decision must be made within 48 hours to detain or release the alien. Aliens paroled or released on bond may be rearrested at any time. Additionally, after a removal order has been issued against an alien, the law provides that the alien subject to a final removal order be removed within 90 days, except as otherwise provided in the statute. Certain aliens subject to a removal order "may be detained beyond the removal period and, if released, shall be subject to [certain] terms of supervision." This provision had been interpreted as permitting indefinite detention where removal was not reasonably foreseeable, but in 2001, the U.S. Supreme Court in Zadvydas v. Davis , interpreted it as only permitting detention for up to six months where removal was not reasonably foreseeable. The INA contains both criminal and civil violations. Historically, the authority for state and local law enforcement officials to enforce immigration law has been construed to be limited to the criminal provisions of the INA. The enforcement of the civil provisions, which includes apprehension and removal of deportable aliens, has strictly been viewed as a federal responsibility, with states playing an incidental supporting role. Although there is debate with respect to state and local law enforcement officers' authority to enforce civil immigration law, it is permissible for state and local law enforcement officers to inquire into the status of an immigrant during the course of their normal duties in enforcing state and local law. For example, when state or local officers question the immigration status of someone they have detained for a state or local violation, they may contact a DHS Immigration and Customs Enforcement (ICE) agent at the Law Enforcement Support Center (LESC). The federal agent may then place a detainer on the suspect, requesting the state official to keep the suspect in custody until a determination can be made as to the suspect's immigration status. However, the continued detention of such a suspect beyond the needs of local law enforcement, and solely designed to aid in enforcement of federal immigration laws, may be unlawful. The law requires the detention of criminal aliens; national security risks; asylum seekers, without proper documentation, until they can demonstrate a "credible fear of persecution"; arriving aliens subject to expedited removal (see the section entitled, "Expedited Removal and Detention"); arriving aliens who appear inadmissible for other than document related reasons; and persons under final orders of removal who have committed aggravated felonies, are terrorist aliens, or have been illegally present in the country. In addition, Section 236 of the INA provides for the mandatory pre-removal-order detention of an alien who is certified by the Attorney General as a terrorist suspect. Thus, it can be argued that the Attorney General and the Secretary of DHS have the discretion to detain any alien who is in removal proceedings, and must detain all aliens who are charged as terrorists, and almost all aliens charged as criminals upon their release from criminal incarceration whether they are released on probation or parole.