More than 20 inmates were injured Thursday, three of them critically, in a riot involving 300 men at the state prison in Lancaster, the worst violence at the lockup since it opened in 1993, officials said.
Guards fired several warning shots and used pepper spray, tear gas and wooden bullets to quell the 15-minute disturbance, which began in a maximum-security yard about 10:45 a.m., according to Department of Corrections officials.
It was unclear what touched off the melee, but it began just as the prison ended a nine-day lockdown stemming from an attack on a guard, officials said.
"If we'd known what it was about, we might have been able to prevent it," said K. W. Prunty, the head of the Department of Corrections Southern California Region. "We will interview virtually every inmate in the facility."
Three inmates were airlifted to local hospitals after suffering deep stab wounds from crude knives made by prisoners, department spokeswoman Terry Thornton said. They were in critical condition but are expected to live, she said.
Two prisoners with less serious knife injuries were taken by ambulance to area hospitals, and others were treated at the prison infirmary for cuts and bruises.
Officials said the fighting began among a dozen white inmates, spread to groups of Latinos and African Americans, then fell along racial lines. Latinos attacked blacks in one part of the yard, while whites and blacks fought in another, the officials said.
Like many state prisons, the 262-acre Lancaster lockup, about 70 miles north of downtown Los Angeles, is overcrowded. It was built to house 2,200 inmates but now holds more than 4,000, in part because of longer sentences imposed under three-strikes laws, officials said.
A fight last year between more than 120 white and Latino prisoners left 10 injured. In 1999, a guard was shot during a brawl between whites and Latinos.
Families planning to visit inmates for the holidays flooded phone lines Thursday to check on their loved ones, officials said. Prunty said the section where the fight took place will be locked down indefinitely, with visiting privileges suspended. He said officials will decide Monday whether to allow Christmas visits.
Shari Johnson, whose 31-year-old son is serving time at the prison, said barring Christmas visits would be a mistake.
"It's gonna be big trouble," she said. "I would not be surprised to see something else happen."
Times staff writer Richard Fausset contributed to this report.