The Friday night melee sent seven inmates to area hospitals with non-life threatening injuries, prison spokesman Lt. Anthony Gentile said Saturday.
There were no fatalities, said spokesman Luis Patino of the California Department of Corrections and Rehabilitation.
No corrections officers were injured.
Five inmates are being treated for gunshot wounds, Gentile told CNN on Saturday. The other two are being treated for injuries that they inflicted on one another, he said.
"At this point we have reviewed some of our surveillance tapes and all we've been able to gather it just erupted all at once," Gentile told CNN. "It doesn't appear to have stemmed from one specific incident, but this is preliminary."
The disturbance appeared to start on the handball court in the main exercise yard, Gentile said.
"Staff were using chemical agents, initially, in an attempt to quell the incident," he said. "The inmates failed to comply with the chemical agents. It was just not affecting them."
"Then we deployed 40 millimeter rubber bullets," he continued. "That didn't stop the fight. It just kept going. Finally we had to resort to (a) more lethal force option... some of the combatants were fired upon and were struck."
It took roughly 45 to 50 prison staff members 30 minutes to contain the riot, Gentile said.
The riot started after dinner, around 7:30 p.m. (10:30 p.m. ET) during the inmates' exercise time. "Basically, it started at the handball court and just gradually migrated to several areas of the main yard to encompass nearly the entire main yard population," Gentile said.
The prison was under lockdown as inmates and staff were interviewed in an attempt to determine the cause of the riot.
Gentile said that he didn't know what charges the riot's "combatants" might face. The prison has "a disciplinary process that could effectively extend their stay in the institution."
Folsom State Prison is about 20 miles from Sacramento. It is California's second-oldest state prison and houses medium-security inmates, according to the department of corrections.