Special Order 40 was designed to reassure illegal immigrants who historically had shied from reporting crimes & assisting police investigations.

From The New York Times:

The Los Angeles Police Department was one of the first in the nation — nearly three decades ago — to institute a procedure that prohibits officers from initiating contact with people for the sole purpose of learning their immigration status. The procedure, known as Special Order 40, was designed in part to reassure illegal immigrants who historically had shied from reporting crimes and assisting police investigations.

But in the context of contemporary immigration politics, the procedure is now perceived in black neighborhoods and beyond as a roadblock to using immigration laws as a tool against Latino gang violence.

Over the last year, law enforcement agencies across the country have begun to revisit policies like Special Order 40 after public pressure over crimes by illegal immigrants.