FARRELL, Pa. (AP) — A tanker truck loaded with more than 6,000 gallons of hydrogen exploded at a metals processing plant, burning the driver, injuring another worker and threatening an even larger storage tank.

The blast occurred after 6 p.m. Sunday at the Duferco Farrell Corp. plant in Farrell, about 60 miles northwest of Pittsburgh near the Ohio border, said Mike Hrycyk, human resources vice president.

The truck from Air Liquide, a Houston-based company that produces and distributes cryogenic liquids and gases, was at the plant offloading its hydrogen into a storage tank when the explosion occurred, Hrycyk told The Associated Press on Monday. The tanker then burned, endangering a storage tank that contained 12,000 to 14,000 gallons of hydrogen.

Plant workers and fire crews worked overnight to pour water on the storage tank until the tanker fire burned itself out about 7:30 a.m. Monday, Hrycyk said.

Officials could not release the names of the driver, who was taken to a hospital in Akron, Ohio, with first- and second-degree burns, or a second driver who suffered some cuts and was treated at Sharon Hospital and released.

The two drivers were members of a tandem team who had filled their tanker at a depot in St. Mary and had stopped to unload some of the nearly 7,000 gallons of liquid hydrogen at the plant, said Air Liquide spokesman George Smalley. The truck had been scheduled to unload the rest at a later stop, Smalley said.

Officials don't know what caused the explosion.

"We've got people on the scene, they arrived last night, and that's one of their priorities — to see what happened," Smalley said.

The steel-processing company converts the metal to coils and employs about 570 workers.

Plant workers were evacuated without incident and overnight production shifts were canceled, Hrycyk said. The plant reopened and was operating normally Monday morning.

Neighbors told The Herald newspaper of Sharon that the explosion shook their homes.

"It was an eight on the Richter scale," resident Dave Gervais said.

The federal Occupational Safety and Health Administration was investigating, which is routine whenever someone is injured in an industrial accident.