Quick search by citation:

28 U.S. Code § 540 - Investigation of felonious killings of State or local law enforcement officers

The Attorney General and the Federal Bureau of Investigation may investigate felonious killings of officials and employees of a State or political subdivision thereof while engaged in or on account of the performance of official duties relating to the prevention, detection, investigation, or prosecution of an offense against the criminal laws of a State or political subdivision, when such investigation is requested by the head of the agency employing the official or employee killed, and under such guidelines as the Attorney General or his designee may establish.