United States v. Lopez 514 US 549 (95).
Now I was familiar with Lopez but never read the whole decision. It is a Commerce Clause case. The CC is the main authority of the federal criminal laws.
fn3 of Lopez:
3 Under our federal system, the " `States possess primary authority for defining and enforcing the criminal law.' " Brecht v. Abrahamson, 507 U. S. ___, ___ (1993) (slip op., at 14) (quoting Engle v. Isaac, 456 U.S. 107, 128 (1982)); see also Screws v. United States, 325 U.S. 91, 109 (1945) (plurality opinion) ("Our national government is one of delegated powers alone. Under our federal system the administration of criminal justice rests with the States except as Congress, acting within the scope of those delegated powers, has created offenses against the United States"). When Congress criminalizes conduct already denounced as criminal by the States, it effects a " `change in the sensitive relation between federal and state criminal jurisdiction.' " United States v. Enmons, 410 U.S. 396, 411-412 (1973) (quoting United States v. Bass, 404 U.S. 336, 349 (1971)).
Federal murder, for example, must have a federal element to it, murder on federal property, arranging a murder by mail, interstate phone call etc.
Kidnapping is a federal offense, but the FBI can only investigate under certain condictions. The law reads IF the victim is missing more than 24 hours, it is PRESUMED they were transported across state lines, that is the federal element.
Federal drug laws are based on the commerce clause also.