From the ATF FAQ:
(O8) May law enforcement officers purchase and possess semiautomatic assault
weapons and high capacity ammunition feeding devices?
Yes. The law provides exceptions for law enforcement officers
purchasing assault weapons and magazines for official use. However, assault
weapons may not be lawfully distributed to, or received or possessed by, an
officer having been convicted of a misdemeanor crime of domestic violence. A
licensee may lawfully transfer these items to a law enforcement officer and
the officer may lawfully receive and possess them if:
(1) the officer is a "peace officer" having the authority to arrest
persons for violations of the law and to obtain and execute search warrants;
(2) the officer is employed by a government agency; and
(3) in the case of a semiautomatic assault weapon, the officer has not
been convicted of a misdemeanor crime of domestic violence.