The single-vehicle crash occurred Thursday night when the hand-crew truck struck a guard rail and overturned in lanes on State Route 241, officials said. At the time, the 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.
Six firefighters were taken to trauma centers in conditions ranging from "stable to critical," Fennessy said at a news conference Friday. Two firefighters were treated at Hoag Hospital Irvine and have since been released, he said.
Of the six taken to trauma centers, two were admitted in critical condition and are in the intensive care unit on Friday, Dr. Humberto Sauri told reporters. One is considered "critical but stable" and the other's condition is "quite critical," he said.
Four of the eight firefighters "are more seriously injured than the others," Fennessy said. "They're in tough shape right now... Pray for our firefighters."
Fennessy called the crash "devastating" and a "huge tragedy for our family."
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.
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.
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.
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."
ABC News contributed to this report.