An explosion rocked a metals plant in west Georgia early Thursday, leaving seven people injured, authorities and witnesses said.

The explosion happened around 8:45 a.m. at G&S Metals in Manchester, a little more than 60 miles south of Atlanta. About 20 workers were inside at the time, Manchester Fire Chief Greg Lehtimaki said.

Truck driver Gary Highley, 62, of Wabash, Ind., said he was switching trailers at the plant when he heard the blast.

"I saw the roof blow up," he said. "That was enough for me to get out of there."

The injured men were all plant employees, Lehtimaki said. Their names were not immediately released.

Three of the men were taken to Grady Memorial Hospital in Atlanta, where spokeswoman Denise Simpson said one patient was in critical condition and two were in serious condition. Four with less serious injuries went to other hospitals, Lehtimaki said.

The cause of the explosion was not yet known. The blast broke natural gas lines, causing a fire. The blaze was extinguished within 15 minutes, Lehtimaki said.

Siding and insulation were blown from the rear of the building, which is made of sheet metal, and the blast apparently left a hole in the structure. Broken glass and yellow insulation were scattered nearby, but the building's frame appeared intact.

A city official said that a half-mile area around the plant was being evacuated. However, by midmorning, spectators lined a roadway behind the plant, and it was unclear if an evacuation order was ever issued.

Nana Harris, 25, said she was sleeping in her house a few blocks from the plant when she was awakened by a loud boom. She initially thought her house was on fire.

"It shook my whole house," she said. "I thought if I opened my bedroom door then I'm going to be seeing fire out front."

A team from the Georgia Emergency Management Agency was on the scene Thursday morning, surveying the damage.

The company buys, sells and processes and converts aluminum scrap, and provides metallurgical services, said Carolyn McKinley, executive director of the Meriwether County Chamber of Commerce. The material is used for auto wheels, brake parts and structural components and other applications in the casting industry.

The plant's parent company, G&S Metal Consultants is a privately held company with its only facilities in Wabash, Ind., and Manchester, Ga.

Ken Rauch, human resources manager at the plant's parent company, said Thursday morning that he and other executives were about to get on a plane to get to the plant.