About 50 inmates were subdued by rubber projectiles and pepper spray. Guards and inmates were injured during the incident.
Just beyond the security checkpoint of Folsom State Prison is where the large riot happened. The incident broke out around 12:30 p.m. in the exercise yard of the Level 4 maximum security area where murderers and gang inmates are, many of them sentenced for life.
Some of the inmates were transferred to Folsom State Prison because the other prisons said they were too difficult to manage, an indication of behavioral problems.
While investigators will have to look back on the security footage, they believe about 50 inmates were involved. Correctional officers used pepper spray, rubber bullets and seven rounds of real bullets from their mini-14 semi-automatic rifle to quell the situation.
Correctional officers only use lethal rounds when someone's life is at risk.
Once the prison was on lockdown, the guards sat prisoners in the yard so they could pat them down for weapons.
"In a maximum-security, inmates commonly make inmate-manufactured weapons," said Sgt. Tony Quinn with Folsom State Prison. "It takes a matter of minutes to make a weapon. You can take a common toothbrush, heat the toothbrush up until the plastic is softened, insert a razor into it and you have a weapon in a second."
At least nine inmates were taken to area hospitals, some with stab wounds. A few prison guards suffered minor injuries.
The prison was the scene of a riot in May. It's unclear if Wednesday's riot is related.
The inmates involved in the incident could face charges from participation in a riot to attempted murder; surveillance video at the prison will help officials sort things out.