(CNN) -- The driver of a produce truck said he accidentally turned onto railroad tracks instead of a highway Tuesday morning and became stuck before the vehicle was hit by an oncoming Metrolink passenger train in Southern California, a fire investigator told CNN.
The incident, which occurred just before 5:45 a.m. between the cities of Oxnard and Camarillo, injured at least 28 people, four of whom had severe enough injuries to need immediate attention, Ventura County Fire Department Capt. Mike Lindbery said.
Joe Garces of the Oxnard Fire Department explained the driver's apparent mistake. He said the 54-year-old driver saw the train, fled and called 911.
Sergio Martinez, a battalion chief for the Oxnard Fire Department, said the driver was found unhurt.
An officer driving near the scene spotted a man walking along a road at least a mile away and stopped to talk to him, quickly deducing that the man was the produce driver, authorities said at a press conference Tuesday afternoon.
Investigators are looking into what caused the man to drive onto the tracks.
The driver, who works for a produce service from Yuma, Arizona, has not been arrested and is cooperating, Garces said.
After the accident, emergency personnel treated people on tarps on a road adjacent to the tracks, video from CNN affiliate KABC showed.
At least eight patients have been taken to Ventura County Medical Center, spokeswoman Sheila Murphy said.
The vehicle at the scene was "fully engulfed," in flames, according to the California Highway Patrol.
Five cars derailed, three of which came to rest on their sides, said Margaret Remmen, a management assistant at the Ventura County Fire Department. Officials have completed a search of the cars, she said.
About an hour after the incident, Metrolink spokesman Scott Johnson, speaking from San Bernardino County, said that Train 102 hit the truck at Rice Avenue.
The cars did not crumple because of "collision energy management technology," in which Metrolink invested after a 2008 Chatsworth, California, crash. In that incident, a freight train collided head-on with a Metrolink commuter train, killing 25 people.
The technology, which causes the crash energy to expand outward instead of inward, probably prevented a "larger scale of injuries," Johnson said.
The train cars are also equipped with windows that emergency personnel can easily remove to evacuate passengers, he said. An hour after the crash, "a vast majority, if not all" of the passengers had been evacuated, and the injured were treated on the scene or transferred to hospitals.
CNN's Jason Hanna, Christie Bear, Eliott C. McLaughlin and Paul Vercammen contributed to this report.