(CNN) -- Witnesses said a FedEx tractor-trailer truck was already in flames when it crossed a median and slammed into a bus carrying students in Northern California, the National Transportation Safety Board said Saturday night.
The driver of a white Nissan Altima that passed the bus just before the crash reported seeing flames coming from underneath the FedEx truck, said Mark Rosekind, an NTSB board member.
The truck clipped the car occupied by Joe and Bonnie Duran before it slammed into the bus Thursday evening, killing 10 people -- five high school students, three chaperones and the drivers of both vehicles. More than 30 people, mostly teenagers, were taken to local hospitals.
Rosekind, speaking at a press conference, said some bus passengers were ejected from the vehicle, including the ones who died.
He said the investigation shows the southbound truck did not brake as it crossed the median at a 10-degree angle and entered the southbound lanes. Investigators found 175 feet of tire marks left by the bus, indicating the driver tried to avoid crashing into the truck.
Investigators will look for devices on those vehicles that might provide information about speeds, Rosekind said.
One such device -- the electrical control module -- was destroyed by fire on the truck, he said. The electrical control module on the bus survived but "we don't know what's actually on there," he said.
The truck transmission might also be examined to help determine that vehicle's speed, he said.
Blood samples should be obtained from the bodies of both drivers, which will provide information about whether drugs or alcohol might have been in their systems, he said. Blood samples also might tell investigators if the drivers had inhaled smoke from the fire, he said.
He stressed that investigators are only collecting information now and aren't coming to conclusions about the cause of the accident.
Bonnie Duran told the CNN affiliate KOVR the truck was on fire before it hit the bus. She said she made a quick decision to swerve to avoid a direct hit from the truck, sending their rental car into a ditch.
"Somehow we survived," she said.
Her husband said it was like a scene from a movie.
"I looked over and saw the FedEx truck coming straight for me," Joe Duran told the affiliate. "It was like a 3-D movie. Imagine a Mack truck coming out of a movie."
The Durans were hospitalized and released.
Students traveling from the L.A. area
The students were traveling from the Los Angeles area to take part in a program at Humboldt State University, which allows prospective attendees to visit the campus.
Their journey ended in the fiery wreck 90 miles north of Sacramento when the truck hit one of three buses taking the teens to the campus in Arcarta.
The students on the buses represented 31 Southern California high schools. They were heading to "Spring Preview Plus," which invites low-income and first-generation prospective students to the university.
As part of the program, students stay in residence halls, attend events and visit with staff and students from a program that helps historically underrepresented students, the university said.
The two other buses in the caravan made it to the university. Those students were placed in dorms, and the university is offering them counseling.