MOUNDSVILLE, W.Va. (AP) – A fire broke out in a 1,000-foot-tall smokestack under construction at a coal-fired power plant Saturday, trapping four contract workers, officials said.

Three workers were plucked off the stack by a helicopter about 10 p.m., after being trapped for about two hours above the flames, said American Electric Power spokeswoman Carmen Prati-Miller. A fourth was missing.

“When the fire occurred, (the rescued) individuals were in the stack,” Prati-Miller said. “They were able to make their way to the top of the stack.”

The fire broke out at AEP’s Kammer-Mitchell plant south of Moundsville, about 68 miles southwest of Pittsburgh.

The three rescued workers were being treated for injuries, but their conditions were not known, Prati-Miller said. The workers were employed by Pullman Power, which has a contract to install a fiberglass lining in the stack, Prati-Miller said. The cause wasn’t immediately known, she said. The plant is being upgraded to bring the power station into compliance with federal air pollution regulations.