At least 42 people, most of them retirees on a day trip, were killed and four others were gravely injured Friday morning after a bus and truck collided and caught fire in southwestern France.

An image released by BFM television showed the carcass of the bus -- a collapsing, charred frame engulfed by smoke. Firefighters fanned out along the narrow road, between a wooded area and an upward slope near the village of Puisseguin, about 35 miles east of Bordeaux.

French President Francois Hollande promised an investigation into the crash, which he called "an immense tragedy." Hollande also said "the solidarity of the whole nation" was with the loved ones of the victims.

Eight people escaped from the bus after the driver opened the door, but others were trapped as the blaze consumed the vehicles, Puisseguin Mayor Xavier Sublett said on i-Tele television.

The mayor said later on RTL radio that the truck driver lost control of the vehicle. The bus driver "tried to avoid it, but the truck came and hit it, and he couldn't do anything except activate the mechanism to open the doors to allow some people to get out," Sublett said.

Other authorities remained cautious about the circumstances of the crash. The top government official for the Aquitaine region, Pierre Dartout, told reporters an investigation is under way, and Interior Ministry spokesman Pierre-Henry Brandet said it was too early to know what exactly happened.

The regional newspaper Sud-Ouest reported that 49 people were on the bus. Sky News reported that the collision occurred on what a local resident described as a dangerous bend in the road. The truck was carrying lumber, which may have fueled the deadly flames, according to BFM television.

"I saw a huge, very large, and very intense plume of smoke," one witness to the crash told Sky."It was astonishing. Then I saw a police car and, straight after, fire engines everywhere. It seemed to me like a war zone and that's not saying it enough."

The bus was traveling from the village of Petit-Palais to the village of Bearn, located in the foothills of the Pyrenees mountains. It had traveled just four miles when the collision occurred. Sky News reported that the mayor of Petit-Palais, Patricia Raichin, had lost three members of her family in the tragedy.

The weather in the region was overcast Friday morning but not rainy.

Friday's disaster is believed to be France's worst road accident since 1982 when 53 people, including 44 children, died in the eastern town of Beaune.