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Witnesses approaching the scene of a bus accident in northern California Thursday that left at least 10 dead and dozens injured, say they saw giant flames shooting towards the sky and at least one victim running while on fire.

A FedEx tractor-trailer veered across a highway median and slammed into the bus on Interstate 5 near Orland, a small city north of Sacramento.

"I just heard this loud boom," Steven Clavijo, 18, a student at West Ranch High School in Santa Clarita who was on the bus, said. "We knew we were in major trouble."

The scene was chaotic. One student sitting in the back row saw the entire front of the tour bus engulfed in flames. He told KNBC.com that passengers towards the back smashed open windows to escape.

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    "Everybody screaming and, like, I was panicking so I just started banging the window to the right," Jonathan Gutierrez, a student, told the station. "Everybody, like three people were trying to fit through the window at one time.”

    The 44 high school seniors onboard the bus-- nearly half from the Los Angeles school district-- were participating in a program that invites prospective low-income or first-generation college students to visit Humboldt State University in far Northern California.

    They were listening to hip-hop and watching movies when the bus swerved sharply from left to right.

    It is believed that the FedEx big rig driver might have been trying to avoid a passenger car that was also involved in the crash. The three cars collided and there was an explosion followed by chaos.

    Towering flames devoured both vehicles after the crash, and clouds of thick, black smoke billowed into the sky until firefighters doused the fire, leaving behind scorched black hulks of metal.

    "We heard an explosion," Luis Lopez, a witness, told Fox40. "We came outside and everything was already on fire."

    Lopez said he heard screams and cries for help and saw a man running with his back on fire. He said emergency crews were able to extinguish the fire on the man's back.

    Authorities did not release the names of all the victims, but the drivers of the delivery truck and the charter bus were killed along with three adult chaperones and five teenage students. The students were described as high school seniors.

    Among the chaperones killed were a Los Angeles man and the woman he had proposed to in Paris at Christmas, his grandmother said Friday.

    Michael Myvett, 29, and fiancee Mattison Haywood were serving as chaperones on the bus when it exploded into flames.

    "He just died," grandmother Debra Loyd told The Associated Press, her voice cracking. "He was my grandson, the greatest grandson any grandparent could ever have."

    She said she got the call from authorities in the late morning Friday, and they asked for dental records to confirm his death.

    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."

    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 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.

    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.

    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."

    California Gov. Jerry Brown issued a statement on the crash late Thursday.

    "As we mourn the loss of those who died, we join all Californians in expressing our gratitude for the tireless work of the Red Cross and emergency personnel who responded bravely to this terrible tragedy," Brown said in a statement.

    Click for more from Fox40.com

    The Associated Press contributed to this report.