It remained unclear whether the FedEx driver was somehow distracted or lost consciousness, or whether a mechanical failure occurred when his truck swerved across the median of Interstate 5 and slammed head-on into the motor coach full of students from the Los Angeles area on Thursday evening.
The California Highway Patrol also raised the possibility that a separate collision on the truck's side of the highway might have been a factor. Early highway patrol accounts of the accident said the truck side-swiped a car after crossing the center divider, before hitting the bus.
Two witnesses, Bonnie and Joe Duran, who were reported to be in the clipped car, told California media outlets that the truck was on fire before the collision.
"I was heading along in the outside lane and I looked over and saw the FedEx truck coming straight for me and he was in flames already," Bonnie Duran told a local CBS television affiliate.
Reuters could not immediately contact the Durans and a California Highway Patrol spokeswoman did not immediately respond to requests for comment.
A powerful explosion unleashed by the impact was heard throughout the nearby community of Orland, about 90 miles north of Sacramento, said Glenn County Sheriff Larry Jones.
Among the dead were the two drivers, as well as five high school students and a college recruiter on their way north to visit Humboldt State University in Arcata, California, as part of a program to help disadvantaged college hopefuls.
More than 30 others were injured in the wreck.
"We don't know whether the FedEx driver had fallen asleep, whether he experienced a mechanical failure with his vehicle or whether there was a separate collision on the southbound side that caused him to lose control," said Lieutenant Scott Fredrick, the lead Highway Patrol investigator.
A team of investigators from the National Transportation Safety Board are likely to remain on-site for about two weeks for the board's independent review of the accident.
Most of the FedEx truck, which was hauling two semi-trailers, was consumed in the fire, Mark Rosekind, an NTSB investigator, said at a briefing late on Friday. But, he added, there were sufficient remains of both drivers for authorities to have collected samples for blood and toxicology analysis.
"So right now, one of the things we're in the process of doing is seeing whether or not those required samples were actually collected," he added.
The fire was so intense that it could be days or weeks before some of the bodies can be identified, and investigators will have to rely on dental records or in some cases DNA testing, he said.
On Saturday, an NTSB spokesman said there was a chance that the fire destroyed any electronic tracking modules on the bus that would have recorded the speed and braking.
TWO OTHER BUSES ARRIVED SAFELY
The stricken motor coach was one of three buses of students traveling from Southern California to participate in a spring break recruitment program at Humboldt State. The two other buses had arrived safely at the campus before the third bus crashed.
Nestled near the redwoods about 100 miles south of the Oregon border, the university every year invites high school seniors from disadvantaged backgrounds or who may be the first in their families to attend college to tour the campus.
The Los Angeles Unified School District, the largest in Southern California, said some of the 19 students from its high schools who were on the tour rode on the ill-fated bus, but it could not say whether any of them were among the victims.
Apart from the driver, the bus was carrying 44 to 48 students and several chaperones, highway patrol spokeswoman Lacey Heitman said. About 34 people were taken by air and land ambulances to hospitals with a variety of injuries, police said.
Among those killed was Humboldt State recruiter Arthur Arzola, 26, who worked for the university out of the Southern California community of Rancho Cucamonga.
A recently engaged couple serving as chaperones were also among the dead, local media reported.
Jonathan Gutierrez, 17, told NBC's "Today" show that after the crash the bus filled with smoke and students broke windows to escape. "It was a very surreal moment," he said.
"All of a sudden I heard people screaming," said Gutierrez, who had been asleep before the impact.
Pictures from the scene showed the bus reduced to a burned-out chassis resting sideways across the highway. Yellow tarps were draped over what appeared to be bodies in the wreckage.