Six inmate firefighters were injured when a flash fire occurred in the back of their crew vehicle while working in the Castaic area Tuesday morning.

The incident was reported about 11 a.m. near the Golden State Highway and Templin Highway, according to the Los Angeles County Fire Department.

“A flash fire occurred in the back of an inmate camp crew vehicle,” Fire Department officials said in a tweet hours after the incident.

A flash fire is a sudden burst of flames typically caused by air mixing with a flammable substance and ignited by a heat source

Battalion Chief Chad Sourbeer called the fire “spontaneous,” which he described as being “a little odd.”

All six victims were taken to hospitals with mild to moderate burns and are in stable condition. One was taken to a critical burn center as a precaution because of where the burns were located, Sourbeer said.

They were in the area conducting an “after action review,” and the vehicle was holding about 10 inmates who are part of a hand crew when the fire occurred, Sourbeer explained.

Authorities initially said the victims worked for the Fire Department, but the AP reported they are inmate firefighters with the California Department of Corrections and Rehabilitation. In a program through CDCR, the inmates train with local firefighters and head to the front lines.

The California Highway Patrol and the Los Angeles County Sheriff’s Department also responded to the scene.

No further details about the incident have been released.

Crews were still in the area conducting the investigation hours later.

“I think that investigation’s going to tell us a lot, and once we come to a conclusion with that investigation, that will help us prevent this from happening again,” Sourbeer told KTLA.