More than a dozen inmates attacked each other with makeshift weapons at the Oklahoma State Reformatory on Monday, leaving two dead and 13 others injured, officials said.
Fights broke out about 12:30 p.m. in recreation yards of two of the medium-security prison's six housing units, said Jerry Massie, a spokesman for the Department of Corrections.
Most of the injured suffered cuts and stab wounds, and the entire incident was over in about five minutes, he said.
Three inmates transported to Jackson County Memorial Hospital were in critical condition, a hospital spokeswoman said.
"One fight started on one housing unit," Massie said. "They could be seen by the inmates on the other rec yard, and that triggered a second round of assaults."
No guards or prison staff members were injured.
Prison officers on the yard ordered the inmates to stop fighting, and the inmates complied, Massie said. Guards did not fire any shots or release any tear gas, Massie said.
The prison remained locked down Monday, with all inmates confined to their cells.
Massie said officials believe home-made knives were used in the attack. The cause of the violence remains under investigation.
Scott Barger, an official with the Oklahoma Public Employees Association, said prisons across the state are chronically understaffed because of low pay and high turnover among prison guards.
About 800 medium-security inmates are housed at the Oklahoma State Reformatory, which is 144 miles southwest of Oklahoma City. There are 200 more minimum-security inmates in a separate facility outside the walls of the prison.