The Department of Corrections said fighting broke out about 8 p.m. Saturday during an outdoor recreation period.
Spokesman Chad Lewis said Sunday about 50 offenders may have been involved but officials are still investigating the cause. Several inmates received minor injuries.
The four units known as the West Complex house about 750 inmates, considered among the most difficult at the Walla Walla facility. Lewis said those units are on lockdown through Sunday, when prison officials will reevaluate the situation.
Visiting on Sunday has been canceled.
The entire prison houses about 2,200 offenders.
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