Arizona ? 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.