Cobb County charged a 36-year-old Atlanta police officer with killing a security guard at a Marietta apartment complex after the guard inexplicably barged into the officer's unit early Saturday.

Police spokesman Sgt. Dana Pierce said officers found security guard Xavier Mallet, 37, dead, lying in the parking lot in front of building No. 6 at the Ivy Ridge Apartment just before 4:30 a.m.

Pierce said investigators were told Mallet started banging on Atlanta police Officer John Kevin Freeman's door and then forced his way inside when Freeman's wife answered.

The wife, whose name was not released, ran while Freeman and Mallet confronted each other inside.

Police said Freeman then retrieved a gun from inside the apartment and shot the security guard as he left the scene.

Freeman is being held without bond on charges of murder and aggravated assault.

A spokesman for the Atlanta Police Department said he did not know how the charges would affect Freeman's job with the department.