A bus ferrying migrant farmworkers from Michigan to Texas ran off a highway and hit an overpass in Arkansas on Friday, ripping off the roof and ejecting passengers onto the interstate. Six people were killed and six injured; the driver survived.

The crash on Interstate 40 in North Little Rock happened at about 1 a.m., in light rain and fog following a heavy storm, but it wasn't immediately known whether weather played a role. The National Transportation Safety Board investigation is initially focusing on the possibility of driver fatigue, though investigators may focus on other issues once they're on the scene, spokesman Eric Weiss said.

Roberto Vasquez, 28, was behind the wheel when the bus ran off the right side of the highway, struck a wall and hit the bridge. Vasquez told investigators his residence is in Arcadia, Florida, but his driver's license listed his address as Monroe, Michigan, Arkansas State Police spokesman Bill Sadler said.

Of those who died, three were thrown from the bus, one was partially ejected and the other two died inside, said Col. Bill Bryant, the head of the Arkansas State Police.

Vasquez has agreed to routine drug and alcohol tests, but there's no indication he was intoxicated, state police Maj. Mike Foster said.

The driver and two other employees of Vasquez Citrus and Hauling, a provider of foreign farm labor through the federal H-2A visa program, had been transporting 19 workers from Monroe, Michigan, to Laredo, Texas.

A woman who answered the phone Friday at the Lake Placid, Florida-based company hung up as The Associated Press sought more information.

Twelve survivors of the crash are from Mexico, according to a list released late Friday afternoon by Arkansas State Police. Three others live in Florida, while police didn't have a residence for another survivor. Their ages range from 19 to 43. The names of those killed are being withheld until all families have been notified, police said.

U.S. Immigration and Customs Enforcement officers were called in, but only to help local authorities communicate with the Spanish-speaking survivors, according to ICE spokesman Bryan Cox. He said his agency was not pursuing any kind of criminal investigation of the people involved.

The bus had just been sold by Jeff Lawson, who owns Continental Charters in Detroit. Lawson told the Arkansas Democrat-Gazette that the buyer "said he needed a second bus to haul people from (Detroit) to Texas... and Florida." The bill of sale and title to the bus, both dated Oct. 31, declared its value to be $8,000.

Michigan Department of Transportation spokesman Michael Frezell said the 1997 Van Hool bus had passed all of its annual inspections since 2009, as required under state law. Frezell said the bus was last inspected in April and that transportation officials did not know what condition it was in when it was sold.

The American Red Cross was providing mental health services to those who escaped injury. "As you can imagine, people are pretty shaken by this," Regional Communications Director Brigette Williams said.

Highway officials said the span remained structurally sound, even though the impact tore off much of the roof, mostly toward the rear of the bus. Traffic was snarled for hours, but the scene was cleared before daybreak.