Updated

Michael Myvett proposed to his college sweetheart on bended knee in front of the Louvre museum in Paris. Four months later, the pair died in a fiery head-on collision between a FedEx semi-truck and a bus that killed eight others in California, including five promising high school students.

Now more than a year later, the California Highway Patrol has finally addressed a still-unanswered question: What caused the crash?

Agency officials faulted the driver of the FedEx semi-truck at a news conference Friday. But they still were unable to say why his big rig veered across an interstate median and into oncoming traffic on April 10, 2014, hitting a bus that was taking high school students to visit a university.

The semi-truck driver, 32-year-old Tim Evans, could have fallen asleep or could have suffered from an undiagnosed medical condition, but his body was too badly burned to tell, Sgt. Nathan Parsons said Friday.

Evans had no alcohol or drugs in his system, and no mechanical problems could be found in either the semi-truck or the bus, Parsons said.

Myvett, 29, and his fiancee, 25-year-old Mattison Haywood, were set to be married this July, said the young woman's mother, Carla Haywood.

Haywood didn't find much solace in Friday's announcement because the investigation still hasn't answered the central question of why the semi-truck left the road with no apparent attempt to hit the brakes or avoid the collision.

"We're constantly wondering what happened, questioning what could have been prevented," said Haywood, 63, of Chino.

But Myvett's grandmother, Debra Loyd, said she's found closure in the investigation.

"It was the driver's fault," Loyd said. "I'm satisfied. For one year we didn't know anything. Now we know something."

She called on FedEx to settle lawsuits stemming from the crash and award damages to survivors and the families of those who were killed.

"C'mon, FedEx. Get it done," said Loyd, 63, of Los Angeles. "Do what you gotta do. It's time."

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

Victims and their families also are suing the bus company, Silverado Stages.

Gaylord Hill, whose now-19-year-old son survived the crash, said he wasn't satisfied with the investigation because without a definitive cause, nothing can be done to prevent a similar crash.

He said he thought the California Highway Patrol did all it could.

"They did the best they could," he said. "There was an explosion and a fire, and all the evidence burned up."

The 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. Myvett and Haywood were two of the chaperones.

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