The Department of Homeland Security's (DHS's) U.S. Immigration and Customs Enforcement (ICE) Enforcement and Removal Operations (ERO) has primary responsibility for locating removable aliens and ensuring that aliens directed to depart from the United States do so. In carrying out its mission, ICE relies, in part, on state and local law enforcement agencies throughout the country to assist it by identifying removable aliens. The prominence of immigration enforcement issues during the 2016 presidential election as well as publicity surrounding crimes committed by some unauthorized aliens have reignited debates over immigration enforcement in the interior of the country. More specifically, concerns have intensified over the level of cooperation shown by some state and local law enforcement agencies in notifying ICE when they have an alien in their custody. One homicide case, the July 2, 2015, slaying of a woman on a San Francisco pier by a reported unauthorized alien with a criminal and deportation history, is particularly noteworthy because the law enforcement agency in question reportedly had not honored an ICE request to detain the criminal alien who, upon his release, subsequently committed the crime. Some jurisdictions, through resolutions, executive orders, or local ordinances, have expressly defined or limited their roles and the activities of their employees regarding immigration enforcement. These have been referred to as "sanctuary" jurisdictions, and their policies range from limiting law enforcement agents (LEAs) from cooperating with ICE in enforcing immigration law to restricting some types of information that can be shared about an alien with federal law enforcement. Given the range of such enforcement policies, there remains no generally agreed upon definition for "sanctuary" jurisdiction. Critics of sanctuary jurisdictions contend that they impede law enforcement's primary mission in ways that could lead to calamitous outcomes (such as the homicide described above) or could encourage illegal immigration. Supporters of sanctuary jurisdictions maintain that they are needed because of resource and legal constraints, the need to avoid the disruption of critical municipal services by diverting local law enforcement personnel to handle immigration enforcement, and community policing concerns. This report examines the interplay between the federal government (i.e., ICE) and state and local jurisdictions in enforcing immigration law, with a specific focus on noncitizens who have been convicted of a crime. It explores federal resources available to state and local law enforcement agencies that cooperate with ICE to enforce immigration law. The report begins by briefly discussing the evolution of cooperation between the federal government and local law enforcement in carrying out federal immigration policy. It then discusses current administrative efforts to involve state and local law enforcement in enforcing immigration law. A brief discussion of resources dedicated to these efforts follows. The report concludes with a discussion of select issues and an analysis of possible policy approaches for Congress. The enforcement of immigration laws in the interior of the United States has long been a controversial topic. Traditionally, the debate emphasized economic and labor market issues, with those concerned about whether unauthorized aliens were depressing wages and taking jobs from native workers pitted against those who argued that foreign labor was critical for certain industries and benefitted the broader economy. After the attacks of September 11, 2001 (9/11), attention refocused on the adequacy of interior immigration enforcement, especially the perceived lack of federal resources. Although ICE has seen an increase in resources to carry out its immigration enforcement responsibilities, the number of ICE agents pales in comparison to the resources available to local law enforcement agencies (LEAs) throughout the country. While immigration enforcement is a federal responsibility, interior enforcement programs that involve cooperation between ICE and state and local law enforcement agencies can allow a relatively small number of ICE agents to leverage much larger numbers of state and local law enforcement agents. ICE operates four major programs that target criminal aliens. The Criminal Alien Program (CAP) serves as an umbrella program for marshaling the agency's resources to identify and remove criminal and other removable aliens. CAP is guided by the Priority Enforcement Program (PEP ), which represents a set of enforcement priorities that describe which foreign nationals should be removed and in what priority order. PEP also comprises a data sharing infrastructure or "interoperability" between DHS and the Department of Justice (DOJ) that screens for immigration violations when individuals are booked into jails. ICE's §287(g) program allows DHS to delegate certain immigration enforcement functions to specially trained state and local law enforcement officers, under federal supervision. The National Fugitive Operations Program (NFOP) pursues known at-large criminal aliens and fugitive aliens outside of controlled settings (i.e., administrative offices or custodial settings). The Criminal Alien Program (CAP) is an umbrella program that includes several systems for identifying, detaining, and initiating removal proceedings against incarcerated criminal aliens. According to ICE, "CAP provides ICE-wide direction and support in the biometric and biographic identification, arrest, and removal of priority aliens who are incarcerated within federal, state, and local prisons and jails, as well as at-large criminal aliens that have circumvented identification." CAP is intended to prevent the release of criminal aliens from jails and prisons by securing final orders of removal prior to the termination of aliens' criminal sentences and by taking custody of and removing aliens who complete their criminal sentences. CAP jail enforcement officers screen people to identify and prioritize potentially removable aliens as they are being booked into jails and prisons and while they are serving their sentences. Such screening covers almost all persons booked into federal and state prisons and local jails. CAP officers search biometric and biographic databases to identify matches in DHS databases and interview arrestees and prisoners to identify potentially removable aliens without DHS records. In addition to onsite deployment of ICE officers, CAP uses video teleconference equipment that connects jails and prisons to ICE's Detention Enforcement and Processing Offenders by Remote Technology (DEPORT) Center in Chicago, IL. CAP also works with state and local correctional departments that provide inmate roster data that ICE then compares to its immigration databases. CAP also manages the Law Enforcement Support Center (LESC), a 24/7 call-center that conducts database checks on the identity and immigration status of arrestees for ICE officers and law enforcement agencies. The Priority Enforcement Program (PEP) has two components. First, PEP includes enforcement priorities that guide immigration enforcement. Second, PEP uses interoperability, a biometric information sharing program between DOJ and DHS that screens for potentially removable aliens when individuals are arrested by state and local law enforcement agents. ICE takes enforcement action against individuals unlawfully present or removable due to a criminal conviction in accordance with its immigration enforcement priorities. Individuals prioritized for removal must either be threats to national security, border security, and public safety (Priority 1); misdemeanants and new immigration violators (Priority 2); or aliens issued final orders of removal on or after January 1, 2014 (Priority 3). Under CAP, when law enforcement agencies book (i.e., take custody of) an arrestee and submit the person's fingerprints to the FBI for a criminal background check, the fingerprints also are automatically checked against DHS's Automated Biometric Identification System (IDENT) database. Potential matches are forwarded to the Law Enforcement Support Center (LESC, see "Criminal Alien Program (CAP)"). ICE agents at the LESC confirm the identity of matched fingerprints and screen their records for immigration violations and criminal histories. If the LESC determines that the arrestee may be a removable alien, it notifies one of ICE's Enforcement and Removal Operations (ERO) field offices for the arresting jurisdiction about the match. After being notified that a removable alien has been arrested, the local ERO supervisor reviews the record and decides how to proceed based on the individual's criminal conviction record, DHS enforcement priorities, and the office's available resources. If the office decides to initiate removal proceedings against an alien, it will typically request to be notified within 48 hours of the individual's scheduled release from law enforcement custody. Under some circumstances, ICE may request that state and local law enforcement agencies detain, for 48 hours, individuals flagged for removal. Since August 2015, ICE also has been using a Request for Voluntary Transfer for certain cases falling outside of the PEP priority categorization. Priority noncriminal subcategories that are not covered under PEP, but for which ICE may seek transfer from cooperative jurisdictions, are aliens apprehended while attempting to illegally enter the country; unlawfully present aliens who have not resided continuously in the United States since January 1, 2014; aliens who have significantly abused the visa or visa waiver programs; and aliens issued final removal orders on or after January 1, 2014. Section 287(g) of the Immigration and Nationality Act (INA) permits the Secretary of Homeland Security to delegate certain immigration enforcement functions to state and local law enforcement agencies. This authority was enacted into law in 1996 but was given new urgency following the terrorist attacks in September 2001. In 2002, the Attorney General proposed an initiative to enter into Section 287(g) agreements with a number of jurisdictions in an effort to carry out the country's anti-terrorism mission. Under these agreements, commonly referred to as Section 287(g) programs, state and local law enforcement officers could be trained to assist ICE with enforcing certain aspects of immigration law. Prior to 2013, the Section 287(g) program encompassed both task force and jail enforcement agreements . However, ICE currently only has jail enforcement agreements with state and local jurisdictions. Under these agreements, specially trained officers within state and local corrections facilities are authorized to identify criminal aliens by interviewing them and screening their biographic information against the same DHS databases used by CAP agents and officers. The LEAs also use ICE's database and the Enforcement Case Tracking System (known as ENFORCE) to enter information about aliens in their custody. LEAs are supervised by CAP officers. As of July 1, 2016, ICE had Section 287(g) agreements with 32 law enforcement agencies in 16 states. At least 1,675 state and local law enforcement officers had completed ICE's four-week Section 287(g) training and were certified to conduct certain immigration enforcement duties.