Ohio ? An explosion at a coal-burning power plant killed a worker delivering pressurized hydrogen and injured nine others Monday, authorities said.

Officials weren’t sure what caused the blast outside the Muskingum River Plant near Beverly, said Vikki Michalski, a spokeswoman for American Electric Power.

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 is more efficient than using air.

Nine workers were treated at hospitals, mostly for cuts and bruises, authorities said.