PARIS — A truck and a bus transporting retirees on a day trip collided and caught fire Friday on a country road in wine country in southwest France, killing 42 people and gravely injuring at least four others, authorities said. It was the nation’s deadliest road accident in years.

An image released by BFM television showed the carcass of the bus — nothing but 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 50 kilometers (30 miles) east of Bordeaux.

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.