An unidentified shooter was reported at the ground shipping facility of an airport in Kennesaw, Georgia, about 30 miles northwest of Atlanta, just before 6 a.m. EST, said Cobb County Police spokesman Mike Bowman.
FedEx employee Liza Aiken told the Atlanta Journal-Constitution that she was correcting addresses on packages when she saw a man dressed all in black and armed with a knife, assault rifle and a cartridge belt strapped across this chest.
"He looked like he was heading into war," she told the newspaper. "As soon as I saw him, I ran the other way."
Michael Hogland, a driver at the facility, told the paper his boss called him early Tuesday and said a man had shot a security guard.
Police did not immediately identify the shooter or his connection to the facility.
Two of the patients taken to WellStar Kennestone Hospital in Marietta were in critical condition and four were listed as stable, a spokeswoman said.
"Some were able to walk off the ambulance and others are in the operating room," said hospital spokesman Tyler Pearson.
A FedEx spokeswoman extended condolences to those affected by the shooting.
"The situation is now stabilized, and we are focused on the needs of our team members and cooperating with the law enforcement investigation of this tragedy," said company spokeswoman Shea Leordeanu.
Hundreds of police from multiple agencies had converged on the scene. Bowman said the area around the building had been secured, and officers were clearing the incident location.
"This is still an active and fluid situation," he said in a statement.