A series of large explosions sent a shower of flaming debris on nearby freeways and buildings near downtown Dallas Wednesday in what became a three-alarm blaze, officials said.

The explosions occurred at a gas vendor at 500 S. Industrial Blvd. and Interstate 35-E in the Trinity River bottoms, fire officials said.

"It's snowing ash," one eyewitness told FOX affiliate KDFW-TV. "It's unbelievable."

Authorities evacuated a half-mile area surrounding the Southwest Industrial Gases, Inc. facility and shut down parts nearby Interstates 30 and 35 as the explosions continued for more than half an hour. Video footage showed numerous small fires burning in the area as stacks of gas cylinders exploded.

Three hours after the explosions started, fire crews were hosing down the charred metal wreckage. About a dozen cars in a parking lot and a grassy highway median were damaged.

The canisters held acetylene and propane gas, said Texas Commission on Enviornmental Quality spokeswoman Andrea Morrow. It wasn't immediately clear what caused them begin exploding around 9:30 a.m. Calls to a phone listing for Southwest Industrial Gases weren't answered.

"I thought it was artillery. It was just coming just boom, boom, boom," said witness Tony Love, a former Army soldier.

At the edge of the evacuation zone is Dallas County's main jail and criminal courts building, but operations continued there uninterrupted, said Deputy Michael Ortiz of the Dallas County Sheriff's Department.

"We are prepared to go into any emergency mode that's needed, but we feel pretty secure here in the jail," Ortiz said.

The Environmental Protection Agency's emergency responders were on the way to the scene, said Dave Bary, a spokesman for the agency's regional office. He said they will assist in monitoring the air, but had no information yet.

Parkland Hospital spokesman Robert Behrens said two people injured by the explosions had been brought to his hospital in serious condition. A third person was taken to Methodist Dallas Medical Center, hospital spokeswoman Sandra Minatra said. She could not release his condition.

About 30 buildings near the blasts were without power and would stay that way until fire crews fully extinguished the blaze, said Carol Peters, a spokeswoman for Oncor Electric Delivery.

Vanessa O'Brien said she was standing in a parking lot a few blocks away when she felt at least 20 vibrations from the explosion.

"We felt the whole building move and the windows rattle," she said.

The Associated Press contributed to this report.