OKLAHOMA CITY - Two inmates were killed and 13 injured in a brawl on Monday in the recreation yard of a medium security prison in Granite, Oklahoma, officials said.

No guards were attacked in the battle at the Oklahoma State Reformatory between prisoners, said Kevin Rowland, chief investigator for the state medical examiner.

"It sounds like it was a pretty big brawl," he said.

All of the dead or injured inmates suffered stab wounds, he said. It was not immediately clear whether the fight was over a disagreement between rival gangs.

At least one of the injured inmates was in critical condition, Rowland said.

Jerry Massie, spokesman for the Oklahoma Department of Corrections, said all inmates were confined to their cells after the brawl.

The prison in Granite, 144 miles southwest of Oklahoma City, houses nearly 1,000 prisoners, Massie said.

"We're operating at 98 percent capacity or above," he said of Oklahoma's prisons.