ORLAND, Calif. -- A new twist was added to the investigation of a crash involving a tour bus and FedEx tractor-trailer that killed 10 people when a couple said the big rig already was on fire when it careened across a median and sideswiped their car before slamming into the bus.

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, said, like the bus, they were northbound on Interstate 5 on Thursday afternoon. Bonnie Duran, who was driving, 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."

Officer Lacey Heitman, a spokeswoman for California Highway Patrol, said she could not confirm whether the truck was on fire before the collision until all evidence was gathered. National Transportation Safety Board spokesman Keith Holloway said the agency is investigating the condition of the truck before the collision, including if it was on fire. FedEx spokeswoman Bonnie Harrison wouldn't comment on the reports the truck was on fire.

When the tractor-trailer collided with the charter bus carrying high school students to a college campus tour in California's redwood county, the vehicles exploded into towering flames and billowing black smoke. 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 in critical condition.

As part of what's expected to be a lengthy and broad investigation, federal transportation authorities are examining whether fire-suppression systems they previously recommended to improve motor coach safety could have allowed more of the 48 bus occupants to escape.

In a briefing at the start of the investigation, the NTSB's Mark Rosekind said his agency will not only look into the cause of the crash, but what regulators can do to stop any similar ones from happening in the future. Fire safety is one of six areas the NTSB plans to investigate, partly because it has been long-standing concern of the agency.

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 must have sophisticated suppression systems to control fires, much as high-rise buildings have sprinkler systems.

"Fire suppression holds the greatest potential for saving lives, reducing costs and minimizing damage," according to a recent NTSB list of its safety priorities for all modes of transportation. Existing fire standards dating to the 1970s apply to small fire sources such as lit cigarettes, but they do not apply to large fires that can start outside the bus.

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 included 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.

The law suggests the department issue new standards in those areas within three years if the secretary of transportation decides they are "reasonable, practicable and appropriate."

So far, the government has not proposed any new standards related to passenger evacuation in event of a fire or other fire-related issues, according to Advocates for Highway and Auto Safety, which lobbied Congress for tougher motor-coach safety standards.

"The legislation includes many mandates to the Transportation Department on many aspects of safety, some of them easy, others not so easy," said Jacqueline Gillan, president of the safety advocacy group. "Nonetheless, they all need to be done, and there have been no regulations even on the easy ones."

The Federal Motor Carrier Safety Administration, which is part of the Transportation Department, recently initiated a two-year contract to develop and evaluate test procedures to assess fire detection, suppression and flammability of exterior materials for motor coaches, the safety advocacy group said. The research project, headed by the Southwest Research Institute, is to focus on engine and wheel-well fires.

A 2012 study commissioned by the agency estimated that 160 bus fires occurred each year from such non-collision related causes and 95 percent did not result in injury or death.

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.

The victims included a recently engaged couple from Los Angeles and a newlywed from Orange County chaperoning the trip. Among the students was an identical twin from Riverside whose sister was on another bus that arrived safely at Humboldt.

Silverado Stages, the San Luis Obispo, Calif.-based company that owns the charter bus involved in the crash, has a strong safety record, and it has said it is fully cooperating with the investigation. It is unclear what sort of fire-safety equipment the bus in Thursday's crash had, and the company couldn't be reached for comment Saturday.

As part of its investigation, the NTSB said it is investigating 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 wide; this one was 60.

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Joan Lowy in Washington and Justin Pritchard in Los Angeles contributed to this story.