"The July 2, 2015, slaying of a woman on a San Francisco pier by a reported unauthorized alien with a criminal and deportation history has reignited the debate over immigration enforcement in the interior of the country. This case is particularly noteworthy because the law enforcement agency in question reportedly did not honor an immigration detainer issued by the Department of Homeland Security's (DHS's) Immigration and Customs Enforcement (ICE). At the end of 2013, noncitizens accounted for 25.2% of the 214,575 individuals incarcerated in federal prisons, 3.8% of the 1,270,807 individuals incarcerated in state prisons, and 6.9% of the entire incarcerated population. In that year, noncitizens represented 7.0% of the U.S. population. Sentencing data indicate that drug offenders accounted for almost 51% of all federal offenders in federal prison at the end of the year in 2012. Forty-five percent of noncitizen federal prisoners were incarcerated for drug offenses at the end of 2012. Although immigration offenders represented almost 12% of all federal offenders incarcerated at the end of 2012, they represented 44% of all federal noncitizen offenders. […] While immigration enforcement is a federal responsibility, efforts have continually been made to use the potential 'force multipliers' offered by local law enforcement. However, in recent years, some jurisdictions have expressly defined or limited their roles and the activities of their employees regarding immigration enforcement. Critics argue that these policies can create 'sanctuary' jurisdictions that ultimately encourage illegal immigration. Supporters maintain that they are necessary because of resource and legal constraints, the need to avoid the disruption of critical municipal services, and human rights considerations."
CRS Report for Congress, R44118