Of course a state can enforce its laws against the Feds.
Local police can issue parking tickets to or tow Federal vehicles, even those with Federal plates.
Federal vehicles must be registered to some state, and must meet the safety/emissions inspections laws of that state (e.g. Federal agencies can't buy non-California certified models to be registered in California). Similarly, states have sued and won Federal agency compliance/cleanup of environmental hazards per state, not Federal, standards law (federal laws may have an impact when it is "federal/military property" such as a state park or military base, but not, say, an FBI Bureau office in a commercial building)
Note: the above apply to Federal agencies (legitimately called "the Feds"), but the same general principle applies to Federal agents ("a Fed")
While a homicide committed by a Federal agent in the commission of his/her duties has Federal implications, it is also a local crime; local police can detain/arrest and interrogate a Federal officer, pending further disposition. Other felonies, short of murder, are more clearly handled by state law, without any question of jurisdiction: drunk driving, theft, etc.