The basics of the law allowing police to be involved in immigration enforcement have been well known, but Judicial Watch has the actual documents:

...According to the documents, the 1996 Immigration and Nationality Act (INA) "authorized the Secretary of Homeland Security to enter into a written agreement to delegate the authority of enforcing federal immigration laws to a state or political sub-division of a state." Through Immigration and Customs Enforcement (ICE), local law enforcement officers can receive immigration enforcement training – called "287(g) cross designation training." The cost for the five-week program is approximately $520 per officer...

The documents note that certain states and localities inquired about the training program, but failed to follow through, including: Arkansas, Colorado, Connecticut, Massachusetts, Michigan, New Jersey, New York, Oklahoma, Pennsylvania, Tennessee, and Virginia.

"Some local law enforcement agencies claim they lack the ability to enforce our nation's immigration laws. These documents prove that claim false," said Judicial Watch President Tom Fitton. "Local communities that want to help enforce immigration laws can do so legally and cost effectively."

For more on Los Angeles' sanctuary law "Special Order 40", see this, this, and this. Here's a relevant excerpt from the 9/11 Commission Staff Report. ICE's fact sheet on this is here: ice.gov/pi/news/factsheets/section287g.htm