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By Tim Stelloh

Five people were missing after an explosion on Monday at an oil and gas drilling rig in Oklahoma, officials said.

Pittsburgh County Sheriff Chris Morris said at a news conference that it wasn’t clear what caused the fire, which authorities were notified of at 8:45 a.m. (9:45 a.m. ET).

The blast occurred outside Quinton, about 70 miles from the Arkansas state line, Morris said.

The five unidentified workers were among 22 people working at the site, he said.

“Seventeen came out,” Morris said. “Five are unaccounted for.”

One of the workers who escaped was medivacked to a hospital to be treated for burns, said Pittsburg County Emergency Management Director Kevin Enloe.

Enloe added on Monday afternoon that a fire was still burning at the well head.

The drilling rig belonged to Houston-based Patterson-UTI Energy, which said in a statement that it had activated emergency response systems and was cooperating with first responders and authorities at the scene.

A message left with the site’s operator, Red Mountain Energy, was not immediately returned.