It is written and posted policy in local jurisdictions nationwide that LEOs will not, can not, must not determine the immigration status of criminals with a view toward turning them over to INS for deportation.
A Mexican National totalled my son's car in an intersection two months ago. Local cops were not allowed to ask him if he was here legally...he was, but that's not relevant. They couldn't ask, and if he was an illegal, they couldn't have done anything about it. This is a local policy, not a national one.