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 weather conditions, 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 yesterday.
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 depending on what the investigation entails."
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.
A 26-year-old man who worked as a university recruiter was the first of the adult victims to be named.
Coroner's officials said Arthur Arzola died at University of California, Davis Medical Centre in Sacramento.
Mr Arzola was an admissions counsellor for Humboldt. In his online biography for the job, he described himself as thoughtful, compassionate and friendly and said he liked to walk on the beach when he visited Humboldt.
A university statement praised him for his passionate commitment to helping low-income and first-generation students get into college.
A Southern California family has said it is desperately trying to locate one of two identical twins who were on the bus.
Miguel Serrato said his sister, 17-year-old Marisa Serrato, has been missing since yesterday evening after the accident.
The other twin, Marisol Serrato, was on a different bus that was not involved in the crash and made it safely to the school today.
Marisol had been accepted to the school, while Marisa was waitlisted. The two Norte Vista High School seniors decided to check out the campus.
The 23-year-old Miguel said the family has not slept since learning about the crash on the news at their father's 65th birthday celebration.
The National Transportation Safety Board has sent a team to investigate the accident. "Every piece of paper associated with this will be looked at," said spokesman Eric M Weiss.
Safety advocates have said rules to make crashed buses easier for passengers to escape have not been implemented despite a recommendation from accident investigators 15 years ago.
The National Transportation Safety Board recommended in February 1999 that federal regulators issue new standards for large buses so that after an accident passengers can easily open windows and emergency exits.