Six people were ejected from a bus and killed Friday when the vehicle hit a pickup and rolled over on the interstate 25 miles south of downtown Phoenix, state police said.

The six were thrown about 10 yards in the accident about 5:30 a.m. on the Gila River Indian Reservation near the community of Sacaton, according to the Arizona Department of Public Safety. Their bodies were covered with blue tarps as investigators combed the wreckage, but were later removed from the scene. Investigators who had suspected another person might have been trapped under the bus learned later in the day that there was no one under the vehicle.

The roof of the bus was crushed and all the windows were knocked out, with one thrown as far as 25 yards.

DPS spokesman Bart Graves said two men and four women, all aboard the bus, were killed. The bus, traveling from the central Mexican state of Zacatecas to Los Angeles, was carrying 22 passengers, and the surviving 16 passengers were all injured. Police said the rollover triggered a second accident when another pickup slowed and was hit by a sedan. One person from the car was taken to a hospital.

The cause of the crash was under investigation.

Nine patients were in critical condition at Phoenix-area hospitals, some with injuries ranging from broken spines and pelvises to head injuries, according to the hospitals.

One patient — the driver of the bus — was in serious condition. An 11-year-old boy was in guarded condition, and four people were in good condition. The condition of two more people at a local hospital was not released.

Alex Porras said his sister, Jasmine Porras, 25, was coming back from a two-week vacation in Mexico when the bus crashed. She was critically injured.

"I was shocked when I got the call," a red-eyed Porras told The Associated Press at Maricopa Medical Center. "We're really worried."

Alex Porras, 28, said his sister, who is married, had just graduated from Arizona State University with a nursing degree and was hoping to start working soon so she could help others.

Police said the bus hit the pickup, veered onto the left shoulder of the road, then overcorrected in the opposite direction and rolled once before landing on its wheels.

The accident scene quickly became a mass of police and rescue personnel as ambulances arrived and medics set up a triage area in the middle of the normally busy interstate, preparing the injured for transport.

Medical helicopters landed on the interstate to airlift the most critically injured to area hospitals.

The bus had entered the United States at El Paso, Texas, and was traveling westbound on I-10.

Graves said the investigation will include whether the driver was fatigued as well as the maintenance history of the bus.

The bus is owned by Tierra Santa Inc. A phone message left by The Associated Press was not immediately returned.

Both westbound lanes of I-10 were closed near the scene of the wreck. Traffic was being rerouted onto State Route 587 until further notice.