Meanwhile, the driver of the bus remained hospitalized in critical condition with skull fractures and other serious injuries, Utah Highway Patrol Lt. Steve Salas said.
The bus was carrying 14 people, including the driver, when it drifted off the shoulder of Interstate 70 and crashed into a ravine about 200 miles south of Salt Lake City.
A passenger told authorities there may have been a medical issue with the driver, but Salas said investigators had not confirmed that report. He also said he did not know the name of the driver or whether he had any previous accidents.
Some of the passengers were unconscious and trapped during the crash and had to be helped from the bus by other passengers. One woman helped drag her husband out of the bus and up a hill to the side of the highway.
Salas said most of the passengers escaped from the wreckage by climbing out of a rear window of the bus, which remained upright as it bridged the ravine. Passengers flagged down a truck driver for help.
“Just looking at the impact itself and the amount of energy that was moving forward when that thing collided with the dirt, it is surprising that we didn’t have more fatalities,” Salas said.
No other motorists saw the crash, and investigators have not yet determined how fast the bus was going before the deadly crash.
Investigators don’t believe it was braking or slowing but instead drifted off the right shoulder of the highway along a curve and traveled about 1,000 feet (300 meters) alongside the interstate before it came to a stop in the steep ravine.
The bus is not equipped with seatbelts for passengers, Salas said. The driver’s seat is the only one on the bus with a seatbelt, but investigators don’t know if the driver was wearing it.
Most of the injured people had neck and back injuries.
Authorities identified the girl killed as Summer Pinzon of Azusa, California. She was sitting next to her mother, who was hospitalized with a leg injury after the crash, Salas said.
Authorities did not release the mother’s name but said the two were headed to California.
The bus driver and two passengers were taken by helicopters in serious condition to hospitals. Other injured passengers were taken to hospitals by ambulances.
Salas did not have an update about how many of the other passengers remained hospitalized.
Lanesha Gipson, a spokeswoman for Greyhound, did not have details Monday about the driver but said the company is cooperating with authorities and conducting its own investigation.