Moundsville, W.Va. ? A fire broke out in a 1,000-foot-tall smokestack under construction at a coal-fired power plant Saturday in Moundsville, W.Va., 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.

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.