No guards or staff were hurt in fighting that began about 10:45 a.m. in a yard at California State Prison, Los Angeles County, said Margot Bach, spokeswoman for the state Department of Corrections. The prison is in the Mojave Desert north of Los Angeles.
Guards fired several warning shots and then used pepper spray, tear gas and wooden bullets on the prisoners, but it was unclear whether any were hit, Bach said.
Two inmates received stab wounds and had collapsed lungs, while two others had "serious slash wounds," she said. They were to be airlifted to local hospitals.
Three other inmates were taken by ambulance to local hospitals for unspecified other injuries.
It was unclear how long it took to quell the riot but "the entire institution ... is on full lockdown," Bach said at midday.
The overcrowded prison has been the scene of previous brawls involving racial unrest but that did not seem to be the case this time, Bach said.
"It was multiracial. There wasn't any group attacking any other groups specifically. They all got into it," she said.
The cause, however, was not immediately clear and interviews of inmates could take several days, she said.
The prison currently houses more than 4,000 inmates in its minimum- and maximum-security wings. That is about 185 percent of capacity, but Bach said it was not immediately clear whether that may have contributed to the rioting.
"There could be any number of factors," she said.
Last year, a racial brawl involving more than 120 Hispanic white inmates left 10 injured. In 1999, an inmate was shot by guards during another brawl between whites and Hispanics.
The 262-acre prison opened in 1993 and has more than 1,200 employees, including more than 800 custody staff.
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