SACATON, Ariz. – The bus that rolled over on a busy interstate outside Phoenix, killing six people and leaving 16 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 early in the morning on the Gila River Indian Reservation near the community of Sacaton, south of Phoenix. Two men and four women were thrown about 10 yards from the bus and killed.

Arizona Department of Public Safety officials said their investigation will include whether the driver was fatigued.

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.

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

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.