"According to the driver's statement, apparently he fell asleep at the wheel when he ran into the back of the 18-wheeler," Trooper Julie Lewis told reporters for CBS affiliate WJTV.
Fifteen people were on the bus when it crashed around 11 a.m. on Interstate 20 in northeastern Louisiana. The truck driver, who had pulled onto the shoulder to check his brakes, suffered minor head and neck injuries.
WJTV video footage of the wreck showed the front right portion of the bus crumpled into the back of the tractor-trailer. Bus seats and bloodstained luggage littered the grass along the highway shoulder.
Thirteen passengers and two drivers were headed toward Vicksburg, Miss., on the second day of what was to have been a 16-day trip sponsored by their Baptist church in Texas, state police trooper Julie Lewis said.
The "senior ambassador tour" was to include visits to a Mercedes-Benz factory in Alabama, the Grand Ole Opry in Nashville and the Civil War battlefield in Gettysburg, Pa. Lewis said each passenger paid $960 for the trip, sponsored by the First Baptist Church of Eldorado, Texas, a town of about 2,000 people some 160 miles northwest of San Antonio.
Lewis said the injured were taken to several hospitals in Mississippi and Louisiana, and at least one was injured critically. The bus driver, Kenneth J. Thomas, 66, of Eldorado, suffered moderate injuries. Identities of the passengers were being withheld while families were notified.
Carolyn Mayo, Eldorado city secretary, said the senior citizens on the bus included retired teachers and a retired nurse.
"This is a real active group — they have lots and lots of active members," said Mayo, adding that the group usually takes two trips each year.
The bus and the truck remained upright on the side of the road after the crash, but the front end of the bus was demolished — pushed up under the rear of the 18-wheeler's trailer, its engine driven back against the steering wheel by the impact. Bales of cotton, seats from the bus, pillows and clothing littered the highway.
Emergency crews said no passenger was ejected but some were thrown partly out of the open side of the bus.
Traffic in the eastbound lanes of I-20 was shut down for about 20 miles between Tallulah and Waverly. One lane was opened after 5 p.m. Monday.