SACRAMENTO (CBS13) — The California Highway Patrol on Friday faulted a truck driver for a fiery head-on collision with a bus carrying high school students to a college visit.

The underlying cause of the crash is the FedEx truck driver, Tim Evans, 32, “allowing his vehicle to travel across the median in an unsafe turning movement,” Sgt. Nate Parsons said. “He could have fell asleep, he could have had an undiagnosed medical condition. We’re unable to prove either.”

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The April 14, 2014, collision occurred in Orland, about 100 miles north of Sacramento. The dead were five high school students from the Los Angeles area, three chaperones, and the drivers of the FedEx tractor-trailer and the bus. The bus was full of prospective Humboldt State University students heading for a campus visit, and two of the chaperones were engaged to be married.

The California Highway Patrol released the results of its investigation at a news conference after the agency met with family members of those killed in the collision.

The agency said it found some evidence of fatigue, but could not conclude either way if that played any role in the crash.

Evans “had sufficient time off,” Parsons said. “That day, he had been working approximately eight hours at the time of the collision. But he had between eight and 10 hours of sleep the night before.”

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Evans was survived by a wife, who did not immediately return a Facebook message on Friday.

Carla Haywood, whose daughter Mattison died on the bus, said the investigation didn’t still address their central question of why the truck driver left the road. “We’re constantly wondering what happened, questioning what could have been prevented,” said Haywood, 63 of Chino.

Mattison and her fiancé, Michael Myvett, who also died in the crash, were chaperones on the trip. It was their second year together accompanying students on a campus trip designed to encourage the enrollment of students with disadvantaged backgrounds who would have been the first in their families to attend college.

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FedEx is reviewing the report and will not comment until the National Transportation Safety Board finishes its separate investigation, company spokesman Jim McCluskey said.

Multiple lawsuits have been filed by victims and their families against FedEx and the bus company, Silverado Stages. What has been unclear ever since the crash is why the semi gradually veered across the interstate median and into oncoming traffic. Investigators previously said they found no evidence that the truck driver attempted to slow down or swerve.

The National Transportation Safety Board released documents earlier this month showing Evans had no drugs or alcohol in his system and was reportedly in good health.

Those records also included uncorroborated witness accounts offering insight into the investigation. One passenger seated three rows behind the bus driver said he saw Evans with his head down and slumped toward the door immediately before the crash. Another driver on the highway said the semi’s left turn signal lit up before it changed lanes and drifted across the median.

A couple in a sedan sideswiped by the truck before the crash reported seeing flames coming out of one of its trailers, but state and federal investigators found no physical evidence to support that statement.

The National Transportation Safety Board could release its final report this summer, an agency spokesman said last week.

Copyright 2015 The Associated Press.