Authorities on Friday were investigating what caused the driver of a FedEx tractor-trailer to cross a grassy center divide and slam the vehicle into a chartered bus carrying high school students on a trip to a Northern California university.

The accident, which killed 10 people and left dozens injured, occurred Thursday evening on Interstate 5 in Orland, a town about 100 miles north of Sacramento. Both the FedEx truck and bus burst into flames, shutting down traffic on both sides of the highway.

Among the dead were five students, the two drivers and three chaperons, according to reports.

At least one bus passenger was in critical condition Friday. A total of 35 people were taken to local hospitals after the crash and were being treated for a range of injuries, including burns, officials said.

“I was surprised that anyone walked away from this,” Highway Patrol spokesman Lacey Heitman said Friday. “The damage was horrendous.”

The National Transportation Safety Board said Friday that it had sent a team to investigate.

The 46 passengers on the bus were high school students and their chaperons from Los Angeles and elsewhere who had been on their way to visit Humboldt State University in Arcata, about 100 miles south of the Oregon border.

John Deasy, superintendent of the Los Angeles Unified School District, said in a statement late Thursday that he did not know how many of the district’s students had been on the tour bus.

“We are providing support at the site,” he said. “We will be providing support at district schools tomorrow morning.”

Before hitting the bus, the FedEx truck sideswiped another vehicle, the Highway Patrol said, but it was not immediately clear what role that may have played in the accident.

The tour bus, owned by a company named Silverado Stages, was one of three buses chartered by Humboldt State to take high school students to campus for orientation activities as part of the university’s Preview Plus event. The other two buses arrived safely.

Humboldt State’s Preview Plus program dates back 20years and has the goal of attracting low-income and first-generation college students to the university. High school seniors in the program visit the college for two days, staying in residence halls while attending the Spring Preview event and meeting with administrators from the Educational Opportunity Program.

“Humboldt State University is deeply saddened by a tragic accident that occurred earlier this evening involving a charter bus filled with prospective students,” Rollin Richmond, the college president, said in a statement Thursday evening. “They were on their way to visit campus for the April 11 Spring Preview.”

Mitchell Huezo said his 18-year-old niece, Angela Corro, had been a passenger on the bus. Corro was in stable condition and receiving treatment for smoke inhalation, he said.

“She couldn’t talk,” Huezo said. “She can’t really breathe because she inhaled so much smoke.”

Bonnie Kourvelas, a FedEx spokesman, declined to answer questions Friday, but the company released a statement saying the company was “deeply saddened by the tragic highway accident” and that it was cooperating with the investigation.

On Thursday evening, Enloe Medical Center in Chico said it had received 11 patients. Two people were in critical condition, two were in stable condition and seven others were still being evaluated, said Denise Atkinson, the hospital’s nursing supervisor.

Five patients sent to Mercy Medical Center in Redding were treated and released late Thursday, hospital spokesman Heather Nichols said.

The remaining survivors were being sheltered overnight at the Veterans Memorial Hall, the Highway Patrol said.

Front Section, Pages 2 on 04/12/2014