Those factors include whether the FedEx driver fell asleep, experienced mechanical failure or lost control because of a separate collision on the southbound side of the freeway.
Authorities also will probe roadway and weather conditions, Lt. Scott Fredrick said at a news conference.
The bus was carrying more than 40 high school students on their way to visit Humboldt State University when the FedEx tractor-trailer swerved across a grassy highway median and slammed into their bus in a fiery wreck Thursday.
The crash occurred on Interstate 5 in Orland, a city about 150 miles northeast 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," Fredrick said. "It could take as long as six months depending on what the investigation entails."
Someone kicked out a window on the bus, and many of those aboard squeezed through and ran for their lives to the other side of I-before the vehicle exploded in flames.
The 44 teenagers aboard, nearly half from the Los Angeles school district, were participating in a program that invites prospective low-income or first-generation college students to visit Humboldt State University in far Northern California.
Steven Clavijo, a high school senior from Santa Clarita who planned to enroll at the school, was trying to catch a nap on the bus when he felt the vehicle begin to shake from left to right and then he heard a loud boom.
"We knew we were in major trouble," he said.
After he escaped, two more explosions soon followed. Clavijo and other survivors looked on, knowing others were still trapped in the inferno.
Both drivers were killed, along with three adult chaperones and five teenage students, according to the California Highway Patrol.
Towering flames devoured both vehicles just after the crash, and clouds of smoke billowed into the sky until firefighters doused the fire, leaving behind scorched black hulks of metal. Bodies were draped in blankets inside the burned-out bus.
Three buses were traveling as a convoy and only one was involved in the crash, said Earl Perkins, assistant superintendent of operations for the Los Angeles schools district.
Nineteen of the students on the bus were from the Los Angeles schools, Perkins said.
The National Transportation Safety Board sent a team to investigate.
"Every piece of paper associated with this will be looked at," said Eric M. Weiss, an NTSB spokesman.
The crash happened a little after 5:30 p.m. on the interstate near Orland, a small city about 100 miles north of Sacramento.
The bus was among three the university had chartered as part of its two-day Preview Plus program to bring prospective students from Southern California and the San Francisco Bay Area to tour the Arcata campus, according to university officials. The other two buses made it to campus, and the university was providing those students with counseling.
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