(CNN) -- The bus that crashed Friday on an Arizona interstate south of Phoenix, killing six passengers, was operating illegally, the U.S. Department of Transportation said.
The company, Tierra Santa Inc., had applied last April for authority to operate as an interstate carrier, but never responded after the department asked it for more information, said DOT spokesman Duane DeBruyne. "They never sent it," he said.
The application was formally turned down on December 14, and the company -- based in Los Angeles, California -- was informed of the denial by certified letter, he said.
No one from the company returned telephone calls from CNN.
The wreck occurred at 5:27 a.m. on Interstate 10 south of Phoenix when the commercial bus rear-ended a pickup truck, according to Arizona Department of Public Safety spokesman Bart Graves.
The bus driver lost control and veered to the left, then to the right and off the road, rolling over at least once while several people were ejected from the bus, another Public Safety spokesman, Robert Bailey, told CNN sister network HLN.
Two men and four women died, the spokesmen said.
Sixteen people were taken to hospitals, including six who were critically hurt, Graves said. Among the injured were a 25-year-old pregnant woman, an 11-year-old boy and a 16-year-old girl, he said.
The crash blocked both westbound lanes of traffic, according to the Public Safety Department, which had no estimate for when the lanes would reopen.
Video from CNN affiliate KNXV-TV showed ladders leaning against the bus's broken windows and emergency crews treating some of the injured near the crash site.
The blue-and-white bus settled in a ditch beside the road, aerial footage showed.
Los Angeles, California-based Tierra Santa owns the bus, which started its trip in central Mexico near Durango, Graves said. The group entered the United States via El Paso, Texas, and the bus driver planned to switch with a relief driver once they reached Phoenix, about 30 miles north of the crash site, Graves said.
The driver survived the crash and is cooperating with investigators, Graves said.
Tierra Santa did not immediately return calls for comment from CNN.
CNN's Melanie Whitley contributed to this report.