One is cellphone video shot by the driver of a Nissan Altima hit by the truck just before it slammed into the bus. The other is footage from a dashboard camera provided by the California Highway Patrol.
"This is critical information for us to understand how the fire progressed and also how the passengers exited," said Mark Rosekind, a member of the NTSB.
More clues may also be available from a black box-type recorder on the bus that collects vehicle data. The FedEx truck also was equipped with a recorder, but it was destroyed in the fire.
"Nothing is ruled out yet," Rosekind said. "We're giving you what we know."
On Sunday, Rosekind provided more information on the driver of the twin-trailer FedEx truck, whom he did not identify. The driver was based in Sacramento and earlier in the day of the accident had taken a load of freight to a town about 50 miles south of the Oregon border. The driver picked up two semi-trailers — one partly loaded, the other empty — and was returning to Sacramento when the accident occurred.
Rosekind said the NTSB was in the process of learning the exact nature of the contents being hauled to determine if the truck was carrying any hazardous material. He said investigators also would look at possible driver "fatigue, distraction and other human performance issues."
"We're going to go over that driver's last 72 hours at least," Rosekind said, noting that investigators would be looking at cellphone use, for example. "In this case, we don't have somebody to interview. And so what we have to do is to rely on other forms of information to tell what's going on."
Another witness who lives next to Interstate 5 said he saw no flames from the truck before the crash and watched the vehicle swerve out of control after trying to move into the fast lane.
Both witnesses said the truck veered sharply from southbound lanes, across the median and into a Silverado Stages charter bus, which carried 48 people, including 44 Southern California high school students. The students were headed to Humboldt State University for an orientation program.
"When they collided, it was boom!" said Ryan Householder, 31, who watched from his home, where he had been mowing his lawn. He said he was haunted by the screams of those who couldn't escape the burning bus.
Rosekind said the truck left no skid marks on either the roadway or the median as it veered into oncoming traffic. In contrast, more than 145 feet of tire marks indicated that the bus driver tried to stop and swerve to the right.
"That driver was clearly reacting to a situation with braking and a driving maneuver," he said.
Rosekind cautioned that it remained too early to tell what prompted the FedEx driver to leave the southbound lanes. The investigator said blood samples had been obtained from the drivers, both of whom died in the crash. The samples will be used to test for alcohol, drugs or medication.
Some students escaped the burning bus by kicking out the windows. But Rosekind said that all but one window on the bus could be used as emergency exits. He said investigators still have to determine how those exits were labeled, as well as what the actual procedure would be to open the windows.
"One of the concerns, of course, is did that work or not? If that was jammed, etc., could that be why we were seeing people kicking out windows?" Rosekind said, noting that video evidence and other information provided by FedEx and the motor coach company would be crucial to answering those questions.
The charter bus also was equipped with a door meant for handicap access, Rosekind said, but it was locked and could be opened only by the driver, so it was unavailable as an emergency exit.
Five students and three adult chaperones were also killed.
Rosekind said investigators are expected to remain on the scene for five to 10 days. Much of the investigation is now shifting to Los Angeles, where officials will be meeting with employees of Silverado Stages, the company that owned and operated the charter bus. Others will interview area students who survived the crash.
It could take months for the board to issue its final report, Rosekind said. A preliminary report will be released in 30 days.