Investigators are probing the cause of a second riot in less than three weeks at California's maximum security Pelican Bay State Prison.
A prison spokesman said violence involving more than 60 inmates broke out in the prison's outdoor exercise yard Friday afternoon.
Two inmates seriously injured in the melee were taken to a nearby hospital for treatment. Two guards trying to quell the disturbance suffered minor injuries.
Pelican Bay spokesman Lt. Ken Thomas said that the "gang-related" fighting began in three separate locations at the same time, which suggests the incident was probably planned in advance.
A Dec. 31 riot involving rival gang members imprisoned at the 3,400-inmate facility left two guards injured and more than 20 inmates hurt.
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