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.