SACATON, Ariz. - The bus that rolled over on a busy interstate outside Phoenix, killing six people and leaving 16 others injured early Friday, was operating illegally, federal transportation officials said.

The operator of the bus - Van Nuys, Calif.-based Tierra Santa Inc. - was told in April and December not "to engage in the interstate transportation of passengers by commercial motor vehicle," a Department of Transportation statement said.

The first notice, sent via certified mail, came just days after the company submitted a passenger carrier application to the department's Federal Motor Carrier Safety Administration. The application was denied on Dec. 14. The department's statement didn't give a reason.

The bus in Friday's accident was traveling from the central Mexican state of Zacatecas to Los Angeles.

It entered the United States at El Paso, Texas, and was traveling westbound on Interstate 10 with 22 passengers when it hit a pickup, veered onto the left shoulder of the road, then overcorrected in the opposite direction and rolled once before landing on its wheels.

The roof of the bus was crushed and all of its windows were knocked out.

The crash occurred about 5:30 a.m. MST on the Gila River Indian Reservation near the community of Sacaton, some 25 miles south of downtown Phoenix. Two men and four women were thrown about 10 yards from the bus and killed.

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.

Arizona Department of Public Safety officials said their investigation will include whether the driver was fatigued, as well as the maintenance history of the bus.

Federal investigators were reviewing Tierra Santa's safety operations at the company's Van Nuys office.

A man who answered the phone Friday at Tierra Santa declined to identify himself, and said in Spanish that the company is meeting with authorities about the crash and he couldn't comment. He declined to answer when asked about whether the company was operating illegally.

The company never had federal operating authority, Department of Transportation spokesman Duane DeBruyne said.

A random federal inspection of one of the company's buses in Texas in August found that the vehicle was operating illegally and wasn't allowed to continue its trip.

The inspection also found that the bus had multiple safety issues, including nonexistent or defective emergency exits, and that the driver didn't have proper identification and didn't speak English.

It wasn't immediately known if that Tierra Santa bus was the same one involved in Friday's accident.

Another random inspection conducted by state officials in Arizona July found that a driver didn't speak English, and a state inspection in New Mexico in September found there was a lighting problem; the drivers were given citations, but neither inspection found that the buses were operating illegally, according to department records.

It's unclear why the company was able to continue operating after the inspections.

"It's so early in the investigation, there's so many things we don't know," DeBruyne said.

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

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, was coming back from a two-week vacation in Mexico. The 25-year-old had just graduated from Arizona State University with a nursing degree and was hoping to start working soon, he said. She was critically injured but her condition was upgraded to serious Friday evening.

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

Both westbound lanes of I-10 were closed for several hours near the scene of the wreck, which quickly became a mass of police and rescue personnel as ambulances arrived.

Medics set up a triage area in the middle of the interstate, and medical helicopters landed on the road to airlift the most critically injured to area hospitals.

Associated Press writers Mark Carlson and Jonathan J. Cooper in Phoenix contributed to this report.