Four inmates died of injuries sustained during an attack or fight over the weekend at an Oklahoma prison, officials said, and investigators are trying to determine what sparked the violence.

The incident that also left four inmates injured happened around 4 p.m. Saturday at the Cimarron Correctional Facility in Cushing, a community between the state's two largest cities, Oklahoma City and Tulsa. The prison houses medium- and maximum-security male inmates.

Three inmates were declared dead on Saturday and the fourth died overnight at a hospital. Terri Watkins, a spokeswoman for the state Department of Corrections, said Sunday that preliminary reports show the first three men who died were fatally stabbed. But Watkins said she didn't know the nature of the injuries to any of the other inmates.

The stabbings apparently happened in quick succession while the inmates at a medium-security wing of the prison were being let out of their cells into the exercise yard, Watkins said.

"The entire stabbing incident happened over a couple of minutes," Watkins said, adding that prisoners were all locked in their cells within 40 minutes of the outbreak of violence.

No prison staff were injured or involved in the attack, Watkins said. None of the prisoners' names were released because their families had to be notified first, she said.

One of the treated inmates was later returned to the prison and the other three remained hospitalized in stable condition Sunday, said Steve Owen, a spokesman for Tennessee-based Corrections Corporation of America, which owns and operates the prison.

Owen said the inmate attacks lasted less than two minutes. He didn't immediately respond to an email Sunday seeking further details.

It is the second major violent outburst in months at the prison.

In June, some 200 to 300 of the prison's roughly 1,600 inmates were involved in a brawl there. Eleven prisoners were taken to hospitals after the fight and the facility was placed on lockdown. The fighting occurred among inmates in three separate housing units.

Watkins said that as a precaution, all Oklahoma prisons have been placed on lockdown, which means that inmates are confined to their cells and their movement inside the facility is severely restricted.

She said two separate investigations are underway — one by the Department of Corrections' inspector general's office and the other by CCA.

Oklahoma has the highest rate of prison homicides in the nation, with state inmates killed at a rate more than three times the national average, according to figures from the Bureau of Justice Statistics.

The figures reviewed by The Associated Press as part of a months-long investigation earlier this year show that there were 39 homicides at Oklahoma prisons between 2001 and 2012, a rate of 14 per 100,000 inmates. The national average is 4 per 100,000.

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