OKLAHOMA CITY — The Oklahoma State Reformatory remained on lockdown Tuesday morning as prison officials began a comprehensive search of the facility one day after two inmates were killed and a dozen injured in a recreation yard melee.

Department of Corrections spokesman Jerry Massie said authorities are looking for any weapons or contraband hidden by inmates at the prison 140 miles southwest of Oklahoma City. Makeshift weapons, including knives, were used in the fighting.

Massie said authorities "have a good idea" of what caused the violence after speaking with inmates, but he wouldn't elaborate. Massie said the purpose of the investigation is to develop information for a possible criminal case, but that it would be up to District Attorney John Wampler to file any charges.

Fights broke out about 12:30 p.m. Monday in recreation yards of two of the medium-security prison's six housing units, Massie said. Most of those 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. Another inmate was reported in stable condition in Mangum City Hospital. Others were apparently brought to Elkview General Hospital in Hobart, but that hospital did not release any information.

Massie said Tuesday that one injured inmate was returned to the prison on Monday and another one likely will return today.

"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 were injured. Prison Warden Eric Franklin referred all questions about the incident to Massie.

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 department did not characterize Monday's violence as a riot, reserving this term for use only when inmates assault staff, try to escape or destroy property.

"I think a riot implies a lot more dangerous situation than what this was," Massie said. "It's not like they took over the facility and burned the place down or assaulted staff."

Scott Barger, an official with the Oklahoma Public Employees Association, which represents state employees including corrections officers, said prisons across the state are chronically understaffed because of low pay and high turnover among prison guards. "It's a ticking time bomb," he said.

In July 2005, a brawl erupted between white and black inmates at the reformatory that left a convicted rapist dead and eight other inmates injured. That fight, which also took place in an exercise yard, ended after an officer in a guard tower fired a warning shot.

About 800 medium-security inmates are housed at the reformatory. There are another 200 minimum-security inmates in a separate facility outside the walls of the prison.