Gunfire was heard inside the building complex that houses CNN's headquarters, and a man was seen being carried out on a stretcher with blood covering his face.
CNN reported that the offices of its Internet operations, CNN.com, were evacuated and that a suspect was in custody. Video footage showed police pointing guns at a man lying on the ground inside a building.
An announcement over the complex's public-address system said there had been gunfire "with potential casualties by the escalators" near the main entrance, facing Centennial Olympic Park.
"I heard four or five shots. I really didn't see it. I got out of there quick," said Jas Stanford, 27, who had been helping take down a stage in the park used for Final Four festivities.
CNN is a unit Time-Warner.