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Five students, three chaperones and the drivers of the bus and FedEx truck were killed, according to the California Highway Patrol and Humboldt State University, which was to host the students' visit.
"All of a sudden I heard people screaming," Jonathan Gutierrez, 17, told NBC's "Today" show. He had been asleep before the impact, he said.
Gutierrez, who suffered facial cuts, said the aisle of the bus filled with smoke and students broke windows to escape. "It was a very surreal moment," he said.
The Los Angeles Unified School District, the largest in Southern California, said 19 students from 16 of its high schools were on the bus tour headed to Humboldt State, but could not say whether any of the students who died were students at district schools.
Among the dead was a Humboldt State recruiter, 26-year-old Arthur Arzola, who worked for the university out of the Southern California community of Rancho Cucamonga, the Sacramento County Coroner said on Friday morning.
The school's website names Arzola as a counselor and recruiter. In a biography on the site, Arzola characterized himself as hard-working, compassionate and friendly, and described the university as offering "incredible opportunities that change the world for the better."
More than 30 people were hurt after the driver of the FedEx truck lost control, jumped a divider on Interstate 5, side-swiped a car and smashed head-on into the bus on Thursday evening, CHP spokeswoman Tracy Hoover said.
"They are traumatized, absolutely," Hoover said of the injured. "Most of them have scratches, cuts, burns, contusions and lacerations - a magnitude of injuries."
About 34 people were taken by air and land ambulances to area hospitals in varying conditions, police said. No one in the car that was side-swiped was killed, though the driver was sent to hospital with unspecified injuries.
The highway was closed in both directions and was not expected to reopen until early Friday.
Apart from the driver, the bus was carrying between 44 and 48 students and several chaperones to the university for a campus tour, CHP spokeswoman Lacey Heitman said.
The crash took place near the city of Orland, 95 miles (150 km) north of Sacramento.
The students, traveling from Los Angeles-area high schools, were part of a program that Humboldt State said "brings low-income and first-generation prospective college students from the Los Angeles and San Francisco areas to HSU's campus."
Pictures from the scene showed the bus reduced to a burned-out chassis resting sideways across the highway. Yellow tarps were draped over what appeared to be bodies in the wreckage.
'HARDLY ANYTHING LEFT'
Hoover described hunks of twisted metal and broken glass and said flames had roared through the vehicles.
"The big rig and the bus were both engulfed in flames. You are talking about two vehicles that are destroyed. There is hardly anything left of the truck," Hoover said.
Two other charter buses that were also carrying students to Humboldt - one from the Los Angeles area and one from the Fresno area - had arrived safely, the university said.
Bonnie Kourvelas, a spokeswoman for FedEx Corp, said the company was aware that one of its trucks was involved in the crash and is "cooperating fully with authorities."
Some students were from Manual Arts Senior High School, Robert F. Kennedy Community Schools and Banning High School, said Los Angeles Unified School District spokesman Tom Waldman.
Humboldt State President Rollin Richmond said students from southern California were to attend a spring preview event on Friday.
"Our hearts go out to those who have been affected, and we are here to support them, and their families, in any way possible," he said in a written statement.