A horrific scene unfolded Thursday afternoon on a highway in northern California, where a FedEx tractor-trailer crossed the median and slammed head-on into a bus. At least 10 people were killed, with dozens more injured.

Authorities say the tractor-trailer driver, who died in the crash, may have been trying to avoid a passenger car that was also involved in the accident. The collision caused an explosion and fire on the bus, which was carrying high school students and chaperones on a visit to Humboldt State University in Arcata.

The crash happened at about 5:30p local time in Orland, a small city about an hour-and-a-half north of Sacramento.

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University police were trying to determine which school districts the students were from and fielding telephone calls from anxious parents, according to a statement on the school's website.

"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.

Los Angeles Unified School District Superintendent John Deasy said that he didn't know how many students from his district were victims of the crash.

A high school senior from Alliance Renee & Meyer Luskin Academy High School in Los Angeles said she and a few of her classmates who were accepted to the university were invited to go on the tour.

Steven Clavijo, 18, a student at West Ranch High School in Santa Clarita, told The Associated Press that he was on the bus during the crash.

For hours during the long drive north, the students -- from several different schools in the Los Angeles area -- watched movies and listened to hip-hop on the radio, Clavijo said.

But just as he was trying to get some sleep in his seat in the back of the bus, he said he felt the vehicle shake from left to right.

"I just heard this loud boom," he said. "We knew we were in major trouble."

Investigators say the truck driver might have been trying to avoid a passenger car that was also involved in the crash, which shut down shut down north- and south-bound traffic on the freeway.

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."

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.

One patient was admitted to the burn unit of University of California, Davis, Medical Center in unknown condition.

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The Associated Press contributed to this report.