The bus was carrying a group from Tijuana, Mexico, and heading home from a snow trip to the Big Bear Lake area of the San Bernardino Mountains 80 miles east of Los Angeles when it crashed into a sedan and pickup truck around 6:30 p.m. Sunday.
The cause of the crash remained under investigation.
Records showed the company that operated the bus had failed more than a third of federal vehicle safety inspections in the past two years.
The bus involved in the crash recorded 22 safety violations in about a year's time, including problems with brakes, the windshield and tires, according to inspection reports posted by the Federal Motor Carrier Safety Administration.
Brake issues were noted in at least three inspections since October 2011.
Barrientos and girlfriend Lluvia Ramirez, who both work at a government hospital in Tijuana, spoke to the AP as they waited outside an emergency room at Loma Linda University Medical Center for word on a friend who suffered a broken neck.
Barrientos believed the bus reached speeds of 60 mph during the descent down the mountain that he estimated lasted five minutes before the collision.
"I saw many people dead. There are very, very horrendous images in my head, things I don't want to think about," he said.
Barrientos said he was uninjured and immediately began searching for Ramirez and the other friend, who were both ejected. After he moved them away from the bus to safety, he assisted the bus driver.
Ramirez suffered bruises and a hairline vertebra fracture.
"I was overwhelmed," she said. "I'm a surgical resident and I usually know how to react, but I was so in shock I didn't know what to do. I just stayed with my friend."
The crash left State Route 38 littered with body parts and debris, and the bus sideways across both lanes with its windows blown out, front end crushed and part of the roof peeled back like a tin can.
The bus was going slowly down the hill and being passed by other vehicles when it suddenly sped up for an unknown reason, according to a person involved in the investigation who requested anonymity because the probe was ongoing.
The bus struck a Saturn sedan — one of the vehicles that had passed it — then spun and rolled, hitting the pickup truck that was heading up the hill.
Smoke was coming from the back of the bus, witnesses said. The bus eventually struck something on the side of the road that righted it and it came to a stop.
The bus traveled about a mile from the point it struck the Saturn until it came to a stop, said California Highway Patrol Officer Leon Lopez.
Investigators will determine if mechanical failure or driver error was to blame. The bus driver, who survived but was injured, told investigators the vehicle had brake problems.
"It appears speed was a factor in this collision," Lopez said.
The National Transportation Safety Board was sending a team to the scene.
Lettering on the 1996 bus showed it was operated by Scapadas Magicas LLC, based in National City, Calif.
Federal transportation records show the company is licensed to carry passengers for interstate travel and that it had no crashes in the past two years.
Stephen Keppler of the Commercial Vehicle Safety Alliance, a group with industry and government members, said buses and trucks average about two violations for each inspection.
Records show the bus involved in the crash was flagged eight times for maintenance problems, as recently as October.
Overall, buses operated by the firm flunked 36 percent of random inspections, the records indicate. That's higher than the national average for similar companies — a 21 percent failure rate.
The California company had an overall "satisfactory" rating from the Federal Motor Carrier Safety Administration — but records show three-quarters of similar companies had better safety records.
No one answered the door at the Scapadas Magicas office in a sprawling complex that houses more than 1,300 storage lockers and about 30 small offices.
Greg Etter, general manager of Acropolis Space Center, said the company didn't run buses out of the facility. He declined to comment further on the tenant.
The bus was carrying dozens of men, women and children who had spent Sunday at a winter recreation area, authorities said.
Crews worked through the night to recover the dead, but one body remained aboard the bus early Monday, said Rocky Shaw, a San Bernardino County coroner's investigator.
Officials hadn't been able to retrieve the body because the front end of the bus was dangling over the edge of the roadside.
Investigators were trying to pick up any personal property to help identify victims.
More than three dozen people were injured, and at least 17 were still hospitalized, including at least five in critical condition. One is a girl.
The pickup driver was in extremely serious condition, said Peter Brierty, assistant county fire chief. Three people were in the Saturn.
Rocky Shaw, San Bernardino County coroner's investigation, said one of the dead victims was a 13-year-old boy. The boy's family from Tijuana was meeting with Mexican Consulate officials after spending the night going from hospital to hospital looking for him.
Jordi Garcia, marketing director of Interbus, said his company rented the bus from Scapadas Magicas, which supplied the driver.
Interbus offers Mexicans near-daily bus tours to the western U.S. from Tijuana. Its office in a Tijuana strip mall displays photographs of some of its destinations, including Hollywood, the Las Vegas Strip and the San Diego Zoo.
There were 38 people aboard the bus that crashed, including the driver and a tour guide, Garcia said. The bus left Tijuana at 5 a.m. Sunday, with the itinerary calling for a return late that night.
"Everything points to faulty brakes," Garcia said.
He said he spoke briefly with his tour guide, who suffered bruises. She told him she heard a loud pop before the crash.
Garcia said he believed all passengers were Mexican citizens and that there were no U.S. citizens aboard.
Big Bear Lake sits at an elevation of 6,750 feet, and the area has ski resorts and other snow play areas.