MARIETTA, Ga. -- Cobb County Police detectives said an Atlanta police officer shot and killed an unarmed security guard at an apartment complex, and that the officer faces charges of murder and aggravated assault.

Officers identified the victim as Xavier Mallet, 37, of Marietta. Officers said for some unknown reason, Mallet awoke a family by banging on their door early Saturday. And when a female resident answered, officers said, Mallet forced his way into the apartment.

The woman's husband, Atlanta policeman John Kevin Freeman, 36, confronted Mallet, then both followed Freeman's wife out the door. Police said Freeman returned to the apartment, got his firearm, then shot and killed Mallet as he was leaving the area.

Saturday night, Atlanta police issued this statement: "The Atlanta Police Department is in full cooperation with the Cobb County Police Department as they conduct their investigation involving off duty Atlanta Police Officer John K. Freeman. The Atlanta Police Department's Office of Professional Standards is also conducting an adminstrative investigation of this incident."