A Greyhound bus rammed into a tractor-trailer truck as the truck pulled back onto Interstate 40 (search) early Friday, killing the bus driver and two passengers, authorities said.

Seventeen other passengers were taken to hospitals, including two who were critically injured, hospital officials said.

Authorities have not released the name of the driver and the two women who died.

"All we can understand now is that the tractor-trailer was in the emergency lane [shoulder] and pulled back onto the Interstate," Tennessee Highway Patrol Sgt. Glenn Taylor said. "We don't know why the bus ran into the truck."

The accident happened in the westbound lanes about 4:30 a.m. just outside this West Tennessee (search) city, authorities said. The weather was dry and clear.

The crash impact pushed the truck across the median and into the eastbound lanes, but no other accidents happened, Taylor said.

The truck driver wasn't injured and was being interviewed by troopers. His name was not released.

Greyhound spokeswoman Lynn Brown said there were 22 passengers and a driver aboard the bus. Three of the 20 who survived the wreck did not seek hospital treatment.

The westbound bus was en route from Louisville, Ky., to Memphis, Brown said. Earlier Thursday, it had left Cleveland. The bus had made its regular stop in Jackson just before the accident happened, she said.

Jackson-Madison County General Hospital (search) received 11 injured passengers, spokeswoman Judy Plum said.

The hospital sent one critically injured passenger to Regional Medical Center in Memphis, a trauma center. Two other passengers remained at General Hospital, one of them in critical condition, and the other eight were treated and released, Plum said.

Six passengers were treated and released at Regional Hospital of Jackson, a spokesman said.

Dallas-based Greyhound has launched its own investigation into the accident, Brown said.

The company didn't immediately have details about the passengers or where they had boarded, said spokeswoman Kim Plaskett. Greyhound doesn't keep a manifest of passengers but uses information collected at ticket sales offices to identify them.