Spencer's Super-Happy Fun-Time Right-Wing Word Pit

My conversation on Fark today with an opponent of AZ Immigration law.

Me:
I have questions...
Who is it that actually enforcing federal law? Since when did we have a Federal Police force? Isn't it the local law enforcement that enforces federal law? Shouldn't the local law enforcement agencies be empowered to enforce federal laws if no federal police force exists?

Him:
U.S. Immigration and Customs Enforcement (ICE)

Often times the local/state police will be brought in to assist these agents, but the operations are initiated at the federal level, as far as I'm aware.

Me:
ICE is on the streets enforcing immigration? I didn't think ICE directly enforced federal immigration law with a presence, but rather partnered with local law enforcement. In fact, the Immigration and Nationality Act, specifically section 287(g) is designed to increase cooperation with state and local agencies.

How does ICE know what operations are able to be initiated if they are not on the ground, directly enforcing immigration status? Wouldn't it be the role and responsibilities of the local enforcement agencies to inform ICE of irregularities so that ICE can initiation operations where appropriate?

Him:
Are they randomly patrolling streets, do you mean? I would say no, but they are certainly "on the streets" when they're acting on information.

As part of the Dept. of homeland Security, I would imagine their information comes from a number of different sources, including local and state police forces and Border Patrol.

Me:
So why should a local agency be disallowed from checking immigration status when say, someone is pulled over for speeding, arrested for a domestic crime, etc? Isn't this where ICE gets a good share its information?