RED BLUFF, Calif. >> Federal investigators said Sunday that they haven’t found physical evidence confirming a witness’ claim that a FedEx truck was on fire before it slammed into a bus carrying high school students, killing 10 people in Northern California.

National Transportation Safety Board member Mark Rosekind said that investigators are not ruling out a pre-impact fire, but a fire expert did not find evidence of flames as the truck crossed a median, sideswiped a Nissan Altima and crashed into the bus.

“This is all preliminary and factual information,” Rosekind said at a news conference. “We are not ruling anything out.”

The bus was carrying 44 Southern California high school students to a free campus tour of Humboldt State University. Many were hoping to become the first in their families to attend college. Five students, three adult chaperones and both drivers died and dozens were injured in Thursday’s collision in Orland, a small city about 100 miles north of Sacramento.

Bonnie Duran, who drove the Altima and survived with minor injuries, told investigators and reporters Saturday that she had seen flames emerging from the lower rear of the truck’s cab as it approached her car. The bus was gutted and the truck was a mangled mess after an explosion sent flames towering and black smoke billowing, making it difficult for investigators to track the source of the fire.

Rosekind said a blood test of the FedEx truck driver could indicate whether he inhaled smoke before his death. A family member told the Sacramento Bee that the truck driver was Tim Evans, 32, of Elk Grove, Calif.

The biggest questions for investigators include why the truck had left its lane and did not leave behind tire marks, suggesting the driver did not brake. The investigation will review maintenance records and the driver’s medical history, experience and potential impairment.

The bus’ black box-style electronic control module was recovered and will be analyzed. The truck’s device was destroyed, but investigators will take other steps to analyze its speed and maneuvering.

Beyond the cause of the crash, the NTSB will examine if any of its safety recommendations could have reduced the death and injury toll.

In this case, the transportation authorities are focusing on seatbelts, escape routes and fire safety, though it has no authority to enforce measures it recommends.

The victims included passengers who were thrown from the bus, a brand new 2014 model that had seatbelts. Under a rule long sought by Rosekind’s agency, all new motor coaches and some other large buses must include three-point lap-shoulder belts by 2016. But Rosekind said it’s difficult to issue guidelines to enforce seatbelt use while they aren’t mandated.

“In the absence of a flight attendant, the likelihood of anyone on a bus buckling is slim,” said Larry Hanley, president of Amalgamated Transit Union representing bus drivers and advocating for policies reducing driver fatigue.

Bodies recovered from the bus were charred beyond recognition. The transportation board has also called for measures to detect and suppress fires and make buses less vulnerable to blazes after a 2005 bus fire killed 23 nursing home evacuees escaping Hurricane Rita in Texas. Rosekind said investigators will examine the materials and design of the bus to withstand fires.

Fire-suppression systems, now under study by the federal government, are designed with blazes that start in engines and wheels. The systems, akin to a hand-held extinguisher automatically dousing the first embers and sparks, aren’t suited for massive blazes following collisions, said Joey Peoples, a vehicle fire safety expert for SP Fire Research.

“Once you have a fire, it’s now simply a matter of how do we buy enough time to evacuate all the passengers,” Peoples said.

Almost every window on the bus involved with Thursday’s crash was available as an emergency exit, Rosekind said Sunday. Students escaped through them before the fiery explosion that devoured the vehicles.

However, safety standards to make large buses easier for passengers to escape after a crash have not been adopted 15 years after accident investigators called for new rules.

The NTSB will also evaluate 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.

Humboldt State University chartered two more buses to bring more than 500 prospective students to the campus for a three-day visit. Those who made it to the university were sent home earlier than scheduled Saturday morning in light of the tragedy.

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Joan Lowy in Washington, D.C., Terry Chea in San Francisco and Justin Pritchard in Los Angeles contributed to this story.