A FedEx spokesman confirmed the shooting but provided no additional details.
"FedEx is aware of the situation," said spokesman Ben Hunt. "Our primary concern is the safety and well-being of our team members, first responders and others affected. FedEx is cooperating with authorities."
Hundreds of police had converged on the scene, and more patients were expected at a hospital that already was treating six people from the incident, local media reported.
"(My boss) called me about 6 o'clock this morning saying there was a guy inside, shot the security guard," Michael Hogland, a driver at the FedEx facility, told the Atlanta Journal-Constitution.