William Maurice Woodard, 26, was arrested at a traffic stop, DeKalb County Deputy Police Chief Mike Burrows said during a news conference.
The DeKalb Sheriff's Office issued warrants Friday charging Woodard and Mario O'Brian Westbrook, 31, with murder. Burrows said authorities are still looking for Westbrook.
DeKalb County Police officers Ricky Bryant Jr., 26, and Eric Barker, 33, were killed early Wednesday in an apparent ambush in a parking lot at the Glenwood Gardens apartment complex.
Two other men already arrested, Herbie DeShawn Durham, 32, and Deanthius Jamal Johnson, 28, were charged with two counts of murder.
Residents say drugs, prostitution and gunfire are common at the apartment complex in a suburb six miles east of downtown Atlanta.
Police have not released a motive for the killings and have declined to say if there are other suspects.
Both slain officers were married fathers and were wearing their uniforms when they were shot. They were hired in December as part-time security to prevent break-ins, said Teofil Taut, the owner of the 176-unit complex who also lives in one of the buildings.