Five people were missing Monday after an explosion at an Oklahoma drilling rig sent plumes of black smoke into the sky, emergency officials said.

The drilling rig in Quinton, about 100 miles southeast of Tulsa, exploded just before 9 a.m. while about two dozen people were working onsite, the Pittsburg County sheriff told KOTV. At least three medical helicopters responded to the explosion.

Seventeen people were accounted for - one of whom was flown off the site for medical attention - but five people remained missing Monday afternoon. The explosion sparked several fires and sent thick black smoke into the air. The derrick, a towering metal structure above the well, also collapsed.

"They have extinguished the secondary fires, the primary fire on the well head is still burning," Pittsburg County Emergency Management Director Kevin Enloe said in a Monday afternoon press conference.

No fatalities were immediately confirmed, Oklahoma Office of Emergency Management spokeswoman Keli Cain said.

The drilling site was being operated by Oklahoma City-based Red Mountain Energy, said Matt Skinner, a spokesman for the Oklahoma Corporation Commission, which regulates oil and gas operators. A cause of the blast was not immediately clear.

"There’s no knowledge of what caused it or what it was," Enloe said. The search for the unnacounted, Enloe said, will continue.

"We have searched the surrounding area and have not found anybody at this time," he said. "I’m not going to presume anything."

Skinner said a company that specializes in rig fires and other well control problems also responded to the blaze.

"Everything is contained and there is no danger to other residents," Enloe said, noting that the closest home was located about a half mile from the rig. Homeowners there, Enloe stated, were told to shelter in place.

"There will definitely be operations continuing overnight and tomorrow," Enloe said, noting that there would be another press conference later Monday.

The Associated Press contributed to this report.