LOS ANGELES (Reuters) - Federal investigators sought on Monday to determine the cause of a bus crash that killed 13 people in California over the weekend, and at least four of those who were injured remained in critical condition, officials said.
The bus was travelling west on Interstate 10 when it rammed into the back of a tractor-trailer early on Sunday near Palm Springs, which is located about 100 miles (160 km) east of Los Angeles, California Highway Patrol division chief Jim Abele told reporters.
The bus appeared to be ferrying casino-goers back to Los Angeles from the Red Earth Casino in Thermal, which is about 30 miles (48 km) southeast of Palm Springs, after a night of gambling, Abele said on Sunday.
Officials from the National Transportation Safety Board were seeking to determine the cause of the crash.
The driver of the bus was among those killed in the crash, which injured 31 people, some of whom had only minor injuries.
Of the 14 people who were taken to Desert Regional Medical Center in Palm Springs, as of Monday morning four remained in critical condition, the hospital said in a statement.
Two other patients remained at Desert Regional, with one in serious condition and one in fair condition; all the others have been released, the hospital said.
The hospital said one patient was transferred to a Los Angeles facility. That person's condition was not immediately known.
Officials from the California Highway Patrol and the National Transportation Safety Board said they planned to release more details about the crash on Monday afternoon.
Representatives from those agencies could not be reached for comment on Monday.
On Sunday, Abele said it was not immediately clear if alcohol, drugs or fatigue were factors in the crash. He said it was not known whether the bus was speeding, but said it was travelling "significantly faster" than the tractor-trailer.
Abele said the operator of the bus, USA Holiday, appeared to be an "owner-operator" company, meaning the driver owned the firm.
Federal safety records listed the company as having only one bus and said it had had no collisions in the last two years.