Of course nothing stops the local police from turning them over to the federal officials, except sanctuary policies. Wasn’t NY City proclaimed a sanctuary city by Rudy? He should know how it works.
No, NYC became a sanctuary city in 1989 under Ed Koch.
Except that the Federal officials don't do anything, and the paperwork and hassle for the locals leads to nothing - except possibly a civil rights lawsuit from the Federal Justice Department.
Local enforcement without Federal commitment to the rule of law is just time wasting.