Investigators do not yet know what caused a tractor-trailer to veer across a motorway reservation and slam into a bus, killing 10 people, the California Highway Patrol has said.
They are looking into various factors, including whether the FedEx tractor-trailer driver fell asleep, experienced mechanical failure or lost control because of a separate collision on the southbound side of the road.
Authorities will also probe roadway and the weather, Lieutenant Scott Fredrick said.
The bus was carrying more than 40 high school students on their way to visit Humboldt State University when the tractor-trailer slammed into it, causing a fiery wreck. The crash occurred near Orland, a city about 150 miles north-east of San Francisco.
“Since these are such in-depth, detailed investigations, we don’t expect to have a final report for a minimum of three months, 90 days,” Mr Fredrick said. “It could take as long as six months.”
Someone kicked a window on the bus out, and many of those on board squeezed through and ran for their lives before the vehicle exploded in flames.
The 44 teenagers aboard, nearly half from the Los Angeles school district, were participating in a programme that invites prospective low-income or first-generation college students to visit the university in far Northern California.
Both drivers were killed, along with three adult chaperones and five teenage students, according to the California Highway Patrol.
Safety advocates have said rules to make crashed buses easier for passengers to escape from have not been implemented despite a recommendation from accident investigators 15 years ago.