Alexander urged police departments to confront bad news and hold themselves accountable to the public.
He was a member of President Barack Obama's Task Force on 21st Century Policing, which recommended training officers on use of force and de-escalation techniques.
Alexander was hired as DeKalb's police chief in April 2013 and became the county's public safety director in February 2014, overseeing police, fire, 911 and emergency services.
He was a finalist for the Chicago police superintendent job last year, but Chicago Mayor Rahm Emanuel instead chose Eddie Johnson, who was the city police department's chief of patrol.
He previously served as the president of the National Organization of Black Law Enforcement Executives, federal security director for the Transportation Security Administration and police chief in Rochester, New York.
Alexander said he wants to spend more time with his aging mother.
Alexander has frequently spoken about the need for increased police transparency.
He told The Atlanta Journal-Constitution in December that police should make body cameras available to the public and post use-of-force information online.
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Dr. Cedric Alexander says police in metro Atlanta constantly train to handle these types of terror attacks.