Administrative law is a branch of civil law. Enforcement of immigration law is handled by various federal agencies (Customs and Border Protection, Immigration and Customs Enforcement, Citizenship and Immigration Services). It isn't handled by local law enforcement, although local law enforcement may become involved for a number of reasons. Enforcement of immigration law is no different than enforcement of environmental law, federal communications regulations, or consumer protection regulations. All such laws are codified outside of the criminal code and are enforced by appropriate, specialized regulatory agencies. As Gator noted, they are therefore civil.