LITTLE ROCK, Ark. (WWJ/AP) — 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.

READ MORE: Science of Weather: O'Shea Solar Park

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. The Mexican government said all 19 workers were from Mexico and that they were headed back to their home country. The other three people on the bus, including the driver, were from Florida, according to Arkansas State Police.

Mexico’s Foreign Relations Department issued a statement expressing condolences to the families of those killed and saying it’s committed to providing assistance to those affected through the Little Rock consulate.

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

READ MORE: Michigan Matters: Motor City's Italian Connection Grows!

Arkansas State Police released a list of the survivors of the crash, but said the names of those killed were being withheld until all families have been notified. Survivors’ ages ranged from 19 to 43.

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 WWJ 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.

MORE NEWS: MSP, Metro Detroit Police Crack Down On 'Move Over' Law

TM and © Copyright 2015 CBS Radio Inc. and its relevant subsidiaries. CBS RADIO and EYE Logo TM and Copyright 2015 CBS Broadcasting Inc. Used under license. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed. The Associated Press contributed to this report.