Two inmates were killed and 13 injured in a riot on Monday at the Oklahoma State Reformatory in Granite, Okla., the Associated Press reports.

Kevin Rowland, chief investigator for the state medical examiner said no guards were attacked in the brawl.

The cause of the riot is not yet known, and it has not been confirmed whether a disagreement between rival gangs started it. All the dead or injured prisoners had stab wounds and at least one of them was in critical condition.

The daily local newspaper, the Oklahoman, reported that eight of the injured prisoners had to be taken to the hospital after the fight.

According to Reuters, Jerry Massie, spokesman for the Oklahoma Department of Corrections, said all the inmates were confined to their cells after the riot. The stabbing occurred in the medium-security area of the prison. 200 inmates are in the minimum security area of the prison. The prison hosts around 1,000 prisoners and, referring to Oklahoma’s prisons, Massie said "We're operating at 98 percent capacity or above."

Warden's Secretary Leda Reese says the situation is now under control.