ORLAND -- The investigation into the fatal bus crash in Northern California took a new twist after two witnesses said the FedEx truck was on fire before it hit the tour bus filled with high school students.

The witnesses said flames were visible from the big rig as it crossed the grassy median on Interstate 5 and hit their car, then the bus.

Bonnie and Joe Duran told TV reporters in Northern California that their Nissan Altima sideswiped by the truck before it collided head-on with the bus.

"It was on fire already," Bonnie Duran said.

Investigators have not publicly responded the Durans' account.

Experts say the probe will be broad and could take months.

"This is a very significant and unfortunate tragedy," said Jim Hall, a transportation safety consultant and former chairman of the National Transportation Safety Board. "The NTSB is going to have its hands full on this one."

Although the cause of the accident has yet to be determined, Hall and other safety advocates said it could focus new attention on the NTSB's efforts to improve bus safety and the behind-the-scenes battle over safety standards for motor coaches and other commercial vehicles.

NTSB officials, who arrived at the crash site Friday morning, will look into the condition of the vehicles involved, the safety of the highway, the records of both carriers, the truck driver's health and whether he was fatigued due to his work schedule or other factors.

Investigators said they would try to recover electronic modules from the FedEx truck that might have recorded the vehicle's performance. Authorities added they will employ "sophisticated surveying and mapping equipment," along with 3-D diagramming, to reconstruct the crash and what led up to it.

At a news conference earlier Friday, federal traffic safety investigators said they would be paying special attention to fire safety issues as well as whether crash victims were able to escape from the burning motor coach.

Students in the bus have said riders broke windows to escape the fire.

"The worst thing for the NTSB is to show up, know that we've issued recommendations from a previous accident where lives have been lost ... and find out `that~ if those recommendations had been closed and enacted, lives could have been saved," said Michael Rosekind, an NTSB board member who traveled with the investigation team to Orland.

He said he expected investigators to remain in the area for one to two weeks, gathering not just physical evidence but seeking out potential witnesses to the crash.

Rosekind cautioned that the federal agency is only beginning its work and has few answers at this point about what happened.

"While on scene, we will not be determining a probable cause or speculating about the probable cause of this accident," he said. He pointed out that there was no hard barrier between the opposing lanes of the freeway.

Those are required only if lanes are within 50 feet of each other. The interstate lanes where the accident occurred were 60 feet apart, making barriers optionaly.

Five students and five adults, including both drivers, were killed and more than 30 people were injured when the southbound FedEx truck crossed the median and hit the bus head-on late Thursday afternoon, officials said. Both vehicles were consumed in flames.

Glenn County Sheriff Larry Jones said some of the victims were so badly burned that it would take medical and dental records to positively identify them.

The bus was one of three in a caravan carrying students to visit the campus of Humboldt State University; two originated from Union Station in downtown Los Angeles and the third from Fresno.

It was not immediately clear which Los Angeles-area students were on which bus.

Students on the two buses from Los Angeles were supposed to be divided into two groups: last names beginning with the letters A to L on one bus, M to Z on another, Humboldt State spokesman Frank Whitlatch said. But the teenagers "did what 17-year-olds do," and switched vehicles to sit with their friends.

Only one victim -- Arthur Arzola, a 26-year-old Humboldt advisor -- has been positively identified by coroner's officials. Relatives and school officials have identified a few of the others who have not been accounted for.

At Norte Vista High School in Riverside, a cloud hung over students preparing for prom night as they worried about the whereabouts of classmate Marisa Serrato, whose family told reporters she has been missing since the crash.

Marisa and her twin sister, Marisol, both seniors, chose to skip the dance to visit Humboldt State University, said a classmate who gave only his first name, Elmer.

"We just don't know if they've found her," Elmer said.

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