The National Transportation Safety Board (NTSB) has launched an investigation into a tour bus crash that killed eight people and injured dozens on a remote California road east of Los Angeles Sunday evening.

"The fact that there were fatalities involved, we will be looking at the maintenance of the bus, the mechanics of the bus and so forth," Keith Holloway, an NTSB public affairs officer, told ABC News. "We don't have a lot of information at this point until we have investigators get into the bus."

The tour bus, on a trip organized by InterBus Tours, was carrying families from Tijuana, Mexico when it crashed at around 6:30 p.m. 80 miles east of Los Angeles. More than three dozen people were injured, and at least 17 were still hospitalized, including at least five in critical condition. While investigators were trying to pick up personal property to help identify victims, crews worked through the night to recover the dead, said Rocky Shaw, a San Bernardino County coroner's investigator.

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," California Highway Patrol spokesman Mario Lopez said.

It's still unclear what exactly caused the fatal collision, investigators said. Holloway said the complete investigation can take up to 18 months.

"It's not unusual to investigate a motor coach accident. Bus operations are one of the most wanted transportation safety improvements," Holloway added.

The company released a statement on Facebook saying, "InterBus Tours and his team are working to support our clients and their families that unfortunately suffered an accident coming down from the mountains of Big Bear in San Bernandino CA." The company added, "We deeply regret what happened."

According to The Associated Press, the bus operator, Scapadas Magicas LLC, had failed more than a third of federal vehicle safety inspections in the last two years. The company had flunked 36 percent of random inspections on its vehicles, which included cases of brake and tire problems.

The crash occurred when the speeding bus rear-ended a Saturn sedan on the mountain road, flipped and hit a Ford pickup truck, said Lopez.

The Associated Press contributed to this report.