Just before the crash, the driver, Norberto B. Perez of San Ysidro, shouted to the passengers that the brakes had failed and urged them to call emergency services, passengers said.
"If it didn't have brakes or did have brakes, it would leave different types of evidence," said Bob Snook, an accident consultant who recently retired from CHP after working on accident investigations for 25 years.
The investigations are "basically car autopsies, because they will go over that bus with a fine-toothed comb," Snook said.
Even skid marks need to be analyzed carefully to determine whether, for example, they were created by the bus trying to turn too quickly or braking hard, Snook said. Investigators found skid marks at the crash site.
A special six- or seven-member CHP investigation team is typically augmented by other specialists from the agency, whose expertise can range from roadway engineering to diesel mechanics, Snook said.
Government records showed the bus recorded 22 safety violations in inspections in the year ending in October, including brake, windshield and tire problems.
"They'll be very interested in prior contacts with this company, what prior violations they've had, which could be bad or extremely minor," Snook said. A violation can be a crack in the corner of a windshield all the way up to a broken wheel or axle falling off.
Chris Medwell, an expert in heavy vehicle accident reconstruction at Bloomberg Consulting in Gulf Breeze, Fla., said investigators typically focus on several issues when examine air brakes following a crash, including wear to parts and an adjustment device that compensates for wear. Filters on air compressors that feed the system can clog, and hoses can leak, among other mechanical problems.
But human error can also be at issue.
As the name implies, air brakes use pressurized air for stopping power, rather than the hydraulic fluid used in car brakes. Heavy weight in a vehicle, combined with an inexperienced driver in rugged terrain, can have risks.
"A lot of inexperienced drivers on long grade will pump the brakes. ... Descending mountain grades is a special skill," Medwell said. "You don't want to apply and release, and apply and release, to maintain a constant speed. That's what taxes the system."
Associated Press writer Julie Watson in San Diego contributed to this report.