Assistance by State and Local Police in Apprehending Illegal Aliens: Memorandum Opinion for the United States Attorney, Southern District of California [February 5, 1996] [open pdf - 85KB]
"Subject to the provisions of state law, state and local police may constitutionally detain or arrest aliens for violating the criminal provisions of the Immigration and Naturalization Act. State and local police lack recognized legal authority to stop and detain an alien solely on suspicion of civil deportability, as opposed to a criminal violation of the immigration laws or other laws. State and local police may detain aliens reasonably suspected of a criminal violation of the immigration laws for periods of as long as 45 to 60 minutes when detentions of that length are necessary to allow for the arrival of Border Patrol agents who are needed for the informed federal disposition of the suspected violations."
United States. Department of Justice: http://www.justice.gov/