Massive flames could be seen devouring both vehicles just after the crash, and clouds of smoke billowed into the sky until firefighters had quenched the fire, leaving behind scorched black hulks of metal. Bodies were draped in yellow plastic inside the burned-out bus.
California Highway Patrol dispatchers said the drivers of the tractor-trailer and bus were among the dead, along with seven other people who were riding on the bus.
The crash happened a little after 5:30 p.m. on Interstate 5 near Orland, a small city about 100 miles north of Sacramento.
The bus was one of two that the admissions office at Humboldt State University had chartered to bring prospective students from Southern California to tour the Arcata campus, Humboldt's Vice President of Administrative Affairs Joyce Lopes said.
University police were trying to determine which school districts the students were from and fielding telephone calls from anxious parents, Lopes said.
"Our hearts go out to those who have been affected, and we are here to support them, and their families, in any way possible," Humboldt State's President Rollin Richmond said in a statement on the school's website.
Humboldt State spokesman Simon Chabel said the school was working to confirm where in Southern California the students on the wrecked bus are from.
Los Angeles Unified School District Superintendent John Deasy told the Los Angeles Times that several students from his district were on the bus but that he did not have an exact count.
"There is still a lot of information to be gathered, and we are working as hard as we can to gather that information and communicate it as best as we can to parents and other family members during this terrible tragedy," he said.
A CHP dispatcher says the bus and truck were on opposite sides of the freeway when the truck crossed the median and slammed into the bus, causing an explosion and fire.
Investigators say the truck driver might have been trying to avoid a passenger car that was also involved in the crash, which shut down north- and south-bound traffic on the freeway.
A first responder who helped set up a triage at the scene said 36 or 37 people received injuries ranging from severe to minor burns, broken legs and noses and head lacerations.
"The victims were teenage kids. A lot of them were freaked out. They were shocked. They still couldn't grasp what happened," Jason Wyman with the Orland Volunteer Fire Department said.
"You hear about it in the news, but you never seen anything like that," he said.
Wyman said when he drove a water truck to the highway, both the bus and truck were fully engulfed in flames, sending thick, dark smoke into the air.
He said he was first focused on putting out the fire, and once it was under control he saw dozens of people walking wounded.
"The first victim that was taken out of the bus was severely burned," Wyman said.
Nine people were taken to Enloe Medical Center in Chico, hospital spokeswoman Christina Chavira said. Two of those patients were listed in critical condition, two in fair condition, and six were in undetermined condition as of 10 p.m. Thursday, said Denise Atkinson, an Enloe nursing supervisor.
Another five were taken to Mercy Medical Center in Redding in fair condition.
Bonnie Kourvelas, a FedEx spokeswoman, said in a statement Thursday night: "Our thoughts and prayers are with everyone involved in the tragic accident on I-5 in California. We are cooperating fully with authorities as they investigate."
Leff reported from San Francisco. Associated Press Photographer Rick Pedroncelli in Orland, Associated Press Writers Channing Joseph in San Francisco and Daisy Nguyen in Los Angeles contributed to this story.