Read the following article:

Gang crackdown raises touchy issue

Can someone please explain to me why a person's immigration status wouldn't be a factor to be considered by the police in the investigation of criminal activity or arrest for crimes? I can see that a local police authority may not be able to charge someone with a violation of federal law, but why would they be barred legally from knowing about such violations? Local and state police agencies provide information and evidence all the time to federal authorities when other federal laws are violated so why is illegal immigration off-limits?