A bus transporting teenagers and young adults back from a church camp nestled in the Southern California mountains lost control, tipping onto its side as it navigated a serpentine mountain road, resulting in 20 individuals being hospitalized.

The vehicle began to tremble and veer uncontrollably, rocking from side to side before ultimately overturning, according to Cyntia Ramirez, a 21-year-old community college student who was a passenger on the bus.

She recounted the harrowing incident while she collected her scattered belongings at the Orange County church that had organized the weekend retreat in the San Bernardino Mountains.

Ramirez shared that a youth leader from the trip assisted her in exiting through a hatch on the roof.

“It was just an unfortunate accident,” Ramirez remarked.

The bus with 36 people on board crashed at a curve on two-lane highway near Running Springs, about 113 kilometres north-east of Los Angeles, the San Bernardino County Fire Department said.

The winding route curves along cliffsides and through forest areas at an elevation of about 1860 metres.

“A total of 26 patients, including three with major injuries, were treated. Of those, 20 were transported to area hospitals, all by ground ambulances,” the fire department said.

The others declined to be transported.

Fire department photos showed paramedics tending to patients, some wearing neck braces, seated in a triage area set up on the highway.

There was no fire or hazardous material exposure.

Authorities have not said what caused the crash.

The bus, carrying adults and teens, was traveling downhill back to Orange County after a three-day youth retreat at Camp Nawakwa near the tiny community of Angelus Oaks in the San Bernardino Mountains, said Jarryd Gonzales, a spokesperson for the Diocese of Orange.

About 125 people including teens, staff and volunteers from Our Lady of Guadalupe Catholic Church in Santa Ana participated in the event, he said.

They were heading home in several vans and the bus that was involved in the accident.

“We extend our deepest gratitude to the first-responder agencies for their prompt and professional response in safely evacuating passengers and ensuring they received proper medical attention,” Gonzales said in a statement.

Youth leader Prisilla Mendoza, 20, said she had dozed off in the moments before the crash.

“I was asleep and I just woke up and I heard everybody screaming,” she said.

Mendoza said she blacked out after the crash and doesn’t remember how she got out of the bus.

Father José Félix Troncoso said a half-dozen patients remained at the hospital for treatment for fractured fingers and at least one head injury.

The route was closed in both directions during the investigation, which was led by the California Highway Patrol.

Troncoso said he made the trek up the scenic mountain road to lead Mass for the group on Sunday. He left at 4pm and got word about the accident around 10pm.

“It’s very beautiful, but getting up there is a process,” Troncoso told reporters in Spanish.