A boiler exploded at a coal burning power plant in Beverly, Ohio, on Monday, killing one worker delivering liquid hydrogen, and injuring nine others, authorities said.

At a bout 9:30 a.m., the delivery driver was unloading hydrogen into cylinders just outside the plant when something triggered the explosion. Plant officials weren't sure whether the truck or hydrogen tubes exploded or what caused it, said Vikki Michalski, a spokeswoman for American Electric Power.

The blast blew out the windows of a nearby building.

"It was real strong," said Doug Burke, who was operating a front-end loader at Reed Minerals, about a half-mile from the plant. "I felt it inside my loader, and I looked up and saw smoke and then fire, and then after that steam."

"If you blinked your eyes you would have missed it," he said.

Although hydrogen gas is highly explosive, it is used to cool steam generators at the plant because it has a high capacity for heat and it is more efficient than using air.

Hydrogen is delivered to the plant about once a week, Michalski said.

The Muskingum River Plant continued to produce electricity after the explosion although the unit near the blast was shut down.

AEP officials said they were investigating the cause. Federal safety officials also were at the plant, which is near the West Virginia border and about 100 miles southeast of Columbus.

Nine workers were treated at three hospitals in Marietta and Parkersburg, W.Va., for mostly cuts and bruises, authorities said. None of the injuries were life threatening, Michalski said.