All of them were students and chaperones on their way to tour several colleges.
California officials say they do not know yet what caused a FedEx tractor-trailer to veer across a grassy highway divider and slam into a bus.
Investigators will try to determine who is at fault.
“We don't know whether the FedEx driver had fallen asleep, whether he experienced a mechanical failure with his vehicle, or whether there was a separate collision on the southbound side that caused him to lose control,” said Lt. Scott Frederick, California Highway Patrol.
Meanwhile officials are left with the grim task of identifying the bodies.
“Due to the nature of the crash and the fact that most of the bodies suffered burns has made our task to identify them much more difficult,” said Richard Warren, Undersheriff.
A group of teenagers, from more than a dozen Los Angeles High Schools, were on a college tour when chaos struck.
This home video captured the horrifying ball of fire that erupted when the two vehicles collided.
High school student Jonathan Gutierrez was sleeping on the bus when he woke to screams. He says what followed was mass panic.
“It was hard to breathe in there. That's when I started panicking. The smoke was everywhere; you could not see where you were going. You just saw the light from the window and you could hear everybody coughing,” said Gutierrez.
Students scrambled to escape through windows only to watch helplessly as many of their friends perished inside.
At least ten are dead, five of them students, and dozens more injured.