RED BLUFF, Calif. - U.S. federal investigators could not corroborate on Saturday a driver's claim that a FedEx tractor-trailer was already on fire before it careened across a freeway median, sideswiped her car and slammed into a bus carrying high school students, killing 10 people in a fiery wreck.
Investigators were still looking for more witnesses to Thursday's crash in California, National Transportation Safety Board member Mark Rosekind said.
They also plan to conduct tests to determine if the truck driver inhaled smoke before the collision and search for evidence of a fire at the crash scene before the vehicles exploded into towering flames and billowing black smoke.
Investigators also found no signs that the truck driver attempted to brake before the crash. They found no tire marks as the truck veered off the freeway and crashed into the bus taking the students to a tour of Humboldt State University in Northern California.
Five students, three adult chaperones and both drivers died. Rosekind said the bus driver had relieved another driver whose shift ended during a stop in Sacramento.
Bonnie Duran, who was driving the car, told KNBC-TV in Los Angeles that flames were coming from the lower rear of the truck cab.
``I just looked to the left, and there it was coming through right at me at an angle. I can tell I wasn't going to outrun him, so I just kind of turned to the right and he hit me,'' she said. ``It was in flames as it came through the median. ... It wasn't like the whole thing was engulfed. It was coming up wrapping around him.''
Initial reports by police indicated the truck swerved to avoid a sedan that was travelling in the same direction in Red Bluff, a town about 100 miles (161 kilometres) north of Sacramento, then went across the median. Officer Lacey Heitman, a spokeswoman for California Highway Patrol, said she could not confirm if the truck was on fire before the collision until all evidence was gathered.
Federal investigators also recovered an electronic control module from the bus, but do not know what information it contains. They were not able to recover such equipment from the FedEx truck tractor, a 2007 Volvo, but may be able to calculate speed and manoeuvring using the transmission and marks in steering box.
But in addition to the cause of the crash, federal transportation authorities are examining whether fire safety measures they previously recommended for motor coaches could have allowed more of the 48 bus occupants to escape unharmed.
Bodies recovered from the bus were charred beyond recognition. Dozens of students had injuries including burns, and several remained hospitalized.
The 44 Southern California high school students on the bus in Thursday's crash, many hoping to become the first in their families to attend college, were on a free trip arranged by Humboldt State University. Among the students was an identical twin from Riverside whose sister was on another bus that arrived safely at Humboldt.
As part of its investigation, the NTSB said it is trying to determine whether the FedEx driver might have fallen asleep or suffered a health problem and whether there were mechanical issues with the truck.
The agency also is evaluating whether there should have been a barrier on the median to help prevent head-on collisions. Barriers are required when medians are less than 50 feet (15 metres) wide; this one was 60 feet (18 metres).
By Fenit Nirappil And Joan Lowy. Joan Lowy reported from Washington, D.C. Justin Pritchard in Los Angeles contributed to this story.