A couple say a FedEx tractor-trailer was already on fire when it careened across a median, sideswiped their car and slammed into a bus carrying high school students, adding a new twist to the investigation of the crash that killed 10 people.

Initial reports by police indicated the truck swerved to avoid a sedan that was traveling in the same direction, then went across the median. There was no mention of the truck being on fire.

But Joe and Bonnie Duran, the Seattle-area couple who were in the car, told KNBC-TV in Los Angeles thatas the truck approached them on northbound Interstate 5, they saw flames coming from the lower rear of the cab.

“I just looked to the left, and there it was coming through right at me at an angle,” said Bonnie Duran, who was driving. “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.”

The couple was not seriously injured.

Officer Lacey Heitman, a spokeswoman for California Highway Patrol, said she could not confirm if the truck was on fire before the collision. National Transportation Safety Board spokesman Keith Holloway said the agency is investigating the condition of the truck before the collision, including whether it was on fire. FedEx spokeswoman Bonnie Harrison declined to comment on the report.

When the tractor-trailer collided with the charter bus carrying high school students to visit a college in northern California, the vehicles exploded into towering flames. Bodies recovered from the bus were charred beyond recognition.

Five students from the Los Angeles area, three chaperones and the truck and bus drivers died in the crash about 100 miles north of Sacramento. Dozens were injured, and several remained hospitalized Saturday, including at least one listed in critical condition.

As part of what’s expected to be a lengthy and broad investigation, 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.

After a 2005 bus fire killed 23 nursing home residents escaping Hurricane Rita in Texas, the NTSB called for safety standards that could make buses less vulnerable to fire, including improved protection of fuel tanks. More recently, the NTSB said buses should have sophisticated suppression systems to control fires, much as high-rise buildings have sprinkler systems.

The NTSB, which investigates accidents and their causes, has no authority to require safety changes it recommends.

But a bill passed by Congress in June 2012 directed the Department of Transportation to conduct research and tests on ways to prevent fires or mitigate the effects, among other safety issues. That includes evacuating passengers, as well automatic fire suppression, smoke suppression and improved fire extinguishers. Representatives of the bus industry told Congress that manufacturers were increasingly and voluntarily adding such features.