"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. [...] ICE [Immigration and Customs Enforcement] has made the removal of certain criminal aliens its top priority. [...] Immigration enforcement is a federal responsibility, but efforts have been made continually to use the potential 'force multipliers' offered by local law enforcement. [...] ICE operates the Criminal Alien Program (CAP), which is guided by the Priority Enforcement Program (PEP), a set of immigration enforcement priorities that describe which foreign nationals should be removed and in what priority order. PEP also employs 'interoperability,' a data sharing infrastructure between DHS and the Department of Justice (DOJ) that screens individuals for immigration-related violations when they are booked by law enforcement jurisdictions. [...] In recent years, some jurisdictions have expressly defined or limited their roles and the activities of their employees regarding immigration enforcement. These have been referred to as 'sanctuary' jurisdictions. [...] Congress may choose to consider several issues, including whether the potentially positive impacts on public safety of state and local involvement in immigration enforcement outweigh the potentially negative impacts on both law enforcement resource utilization and community relations within such jurisdictions; and whether increasing law enforcement funding or tying the provision of certain federal grants to greater cooperation with federal immigration enforcement agencies--or a mix of both approaches--would yield the greater cooperation proponents seek."
CRS Report for Congress, R44118
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html