Two people were killed and three more were injured in a drilling rig fire in Coalgate, Oklahoma, late Thursday night.

The Oklahoma Fire Marshal’s office and the US Occupational Safety and Health Administration are investigating the cause of the explosion.

Investigators from both agencies were at the scene Friday afternoon, said Sam Schafnitt, chief of operations for the fire marshal’s office. He said the investigation is still in its early stages and may be wrapped up by Monday.

OSHA did not immediately return phone messages seeking comment.

The explosion occurred around 11:45 p.m. Thursday on an oil rig west of Coalgate. The state Medical Examiner’s Office identified the dead as 26-year-old Gary Keenen of Ada and Kelsey Bellah, 27, of Tulsa.

Two of the injured workers were in critical condition at Baptist Burn Center in Oklahoma City, and the third suffered burns on both hands, Schafnitt said. He said authorities are not releasing the workers’ names until their families are notified.

The Oklahoman reported that all five men worked for Dan D Drilling, a Lamont-based drilling contractor active in Oklahoma, Kansas and the Texas Panhandle. The rig was reportedly owned by the Amarillo, Texas-based company Pablo Energy.

Details for this story were reported by The Ada News in Ada, Oklahoma, and The McAlester News-Capital in McAlester, Oklahoma.