The single-vehicle crash occurred Thursday night when the hand-crew truck struck a guardrail and overturned in lanes on State Route 241, officials said. At the time, eight firefighters were returning from a 12-hour shift on the front lines of the massive Airport Fire.
A vehicle ahead of the fire truck swerved to avoid a ladder and the truck swerved too, striking the guardrail on the right shoulder and overturning multiple times, according to the California Highway Patrol.
6 OC firefighters remain hospitalized, 2 critical, after crash while returning from Airport Fire
Six firefighters were taken to trauma centers in conditions ranging from "stable to critical," Orange County Fire Authority Chief Brian Fennessy said at a news conference Friday.
"I don't know this for sure, but I'm confident, knowing the supervisors and those that manage these crews, that the crew was seat-belted, which I'm sure was very helpful," he said.
Two firefighters who were treated at Hoag Hospital Irvine were released Thursday night and two others were released Friday evening.
The other four firefighters remain hospitalized - two are at Mission Hospital and two are at Orange County Global Medical Center. Fennessy said they're "more seriously injured than the others," calling the crash "devastating" and a "huge tragedy for our family."
"As you can imagine how traumatic it would be to see your fellow firefighters that injured in the middle of the freeway," he said.
6 OC firefighters remain hospitalized, 2 critical, after rollover crash on Irvine freeway
The crash happened on the 241 near the Tomato Springs Toll Plaza in the Irvine area shortly before 7 p.m. Such wildfire hand crews are made up of 20 firefighters. Fennessy said, "What I've heard was that the crew carrier, you know, the crew buggy, as we call them, did swerve for whatever reason, and did roll several times."
"They're heavy vehicles," he said. "They're very top-heavy. So it wouldn't take much, you know, at speed, you know, for them to roll over."
While the crew carrier is in motion, Fennessy said at least one crewmember is typically seated in the front, with the rest seated in the rear. The location of the crash is to the west of the Airport Fire, which has burned more than 23,000 acres since last week.
Firefighters have been battling the Airport Fire "non-stop" since Sept. 9, Fennessy said. He said this team of firefighters was responsible for removing fuel from the path of fire. The injured firefighters are younger members who typically attack wildfires on foot, using tools to clear brush.
Signage on the truck indicated it was an Orange County Fire Authority vehicle for Santiago handcrews.
The crash remains under investigation by the highway patrol.
Meanwhile, the Orange County Professional Firefighters Association has launched an online relief fund to help raise money for the firefighters. You can also donate to their GoFundMe here.
"I ask everyone to pray for the firefighters, pray for the families," Fennessy said.
ABC News contributed to this report.