* Truck loses control, smashes head-on into bus
* High school students were going to visit university
* More than 30 injured in crash north of Sacramento (Recasts with details on investigation)
By Sharon Bernstein
SACRAMENTO, Calif., April 11 (Reuters) - A probe into a California crash that killed 10 people focused on Friday on whether the truck driver lost consciousness, his vehicle had mechanical failure or another collision caused him to swerve across a median and slam into a bus full of students.
Those killed in Thursday's crash include five high school students, the drivers of both vehicles and a college recruiter on their way to Humboldt State University in Northern California as part of a program to help disadvantaged college hopefuls.
The powerful explosion that followed the collision was so loud it was heard throughout the nearby community of Orland, about 90 miles (145 km) north of Sacramento, said Glenn County Sheriff Larry Jones.
"We don't know whether the Fed-Ex driver had fallen asleep, whether he experienced a mechanical failure with his vehicle or whether there was a separate collision on the southbound side that caused him to lose control," said Lieutenant Scott Fredrick, lead investigator for the California Highway Patrol.
The resulting fire was so intense that it could be days or weeks before some of the bodies can be identified, and investigators will have to rely on dental records or in some cases DNA testing, he said.
Three buses of students were traveling north from the Los Angeles area to participate in a spring break recruitment program at Humboldt State University.
Tucked into the redwoods near the Oregon border, the university every year invites high school seniors from disadvantaged backgrounds or who may be the first in their families to attend college to tour the campus.
The Los Angeles Unified School District, the largest in Southern California, said 19 students from 16 of its high schools were on the bus tour, but could not say whether any of the victims were students at district schools. The school district said several of its students were on the ill-fated bus.
Among the casualties was Humboldt State recruiter Arthur Arzola, 26, who worked for the university out of the Southern California community of Rancho Cucamonga, the Sacramento County Coroner said on Friday morning.
The school's website describes Arzola as a counselor and recruiter. In a biography on the site, Arzola characterized himself as hard-working, compassionate and friendly, and described the university as offering "incredible opportunities that change the world for the better."
More than 30 people were also hurt after the driver of the FedEx truck lost control, jumped a divider on Interstate 5, side-swiped a car and smashed head-on into the bus on Thursday evening, CHP spokeswoman Tracy Hoover said.
"They are traumatized, absolutely," Hoover said. "Most of them have scratches, cuts, burns, contusions and lacerations."
Jonathan Gutierrez, 17, told NBC's "Today" show that after the crash the bus filled with smoke and students broke windows to escape. "It was a very surreal moment," he said.
"All of a sudden I heard people screaming," said Gutierrez, who had been asleep before the impact.
Apart from the driver, the bus was carrying 44 to 48 students and several chaperones, CHP spokeswoman Lacey Heitman said. About 34 people were taken by air and land ambulances to hospitals with a variety of injuries, police said.
Pictures from the scene showed the bus reduced to a burned-out chassis resting sideways across the highway. Yellow tarps were draped over what appeared to be bodies in the wreckage.
"The big rig and the bus were both engulfed in flames. You are talking about two vehicles that are destroyed. There is hardly anything left of the truck," Hoover said.
Bonnie Kourvelas, a spokeswoman for FedEx Corp, said the company was aware that one of its trucks was involved in the crash and is "cooperating fully with authorities."
Humboldt State President Rollin Richmond said students from southern California were to attend a spring preview event on Friday.
"Our hearts go out to those who have been affected, and we are here to support them, and their families, in any way possible," he said in a written statement. (Reporting by Sharon Bernstein in Sacramento, Dan Whitcomb in Los Angeles and Eric Johnson in Seattle; Additional reporting by Laila Kearney in San Francisco and Colleen Jenkins in North Carolina; Editing by Cynthia Johnston, Richard Chang and Gunna Dickson)