A fourth suspect was arrested Friday in the killings of two police officers gunned down as they worked as off-duty security guards at an apartment complex.

The DeKalb County Sheriff's Office arrested Mario O'Brian Westbrook, 31, Friday afternoon, WSBTV reported.

This came several hours after authorities arrested William Maurice Woodard, 26.

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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.

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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.

The Associated Press contributed to this report.