RED BLUFF, Calif. — Federal investigators could not corroborate on Saturday a driver’s claim that a FedEx tractor-trailer was already on fire when 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 the Thursday crash, 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 examine crash scene evidence for clues of a fire 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 the truck left no tire marks after it veered off the southbound lanes of Interstate 5 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.

Joe and Bonnie Duran, the Seattle-area couple who were in the car, said, like the bus, they were heading north on Interstate 5 about 100 miles north of Sacramento on Thursday afternoon. Bonnie Duran, who was driving, told federal authorities and 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 told the television station. “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 made no mention of a fire before the crash. Officer Lacey Heitman, a spokeswoman for California Highway Patrol, said she could not confirm if Duran’s account is accurate 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 maneuvering using the transmission and marks in steering box. They are also investigating if the drivers were impaired.

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. More than 500 students, including about 40 from the Los Angeles area, were sent home earlier than scheduled Saturday morning in light of tragedy.

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.

Fire safety is one of six areas the NTSB plans to investigate, partly because it has been longstanding concern of the agency.

After a 2005 bus fire killed 23 nursing home evacuees 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 says buses must 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 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.

National Highway Traffic Safety Administration has commissioned research on fire safety measures, and new rules could be proposed by 2015. 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.”

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 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 wide; this one was 60.