(CNN) -- Several people were ejected from a bus when it crashed on an Arizona interstate Friday, killing six passengers, the Arizona Department of Public Safety said.
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.
Watch KPHO-TV's slide show from the scene
Fifteen of the 21 bus passengers were transported to hospitals, including five who suffered serious injuries and were taken by helicopter, Graves said. Two 11-year-olds -- a boy and a girl -- are among the injured.
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.