A bus carrying 40 elementary school students home from a trip to the French Alps plunged down a wooded slope into a creek Saturday, leaving the driver and his partner hospitalized with serious injuries, authorities said.

Eighteen children and one other adult were treated for minor injuries, said Fabien Mulyk, mayor of the town of Corps, where the incident happened. “They are all doing well,” he told The Associated Press.

The regional prosecutor has opened an investigation. The mayor said the “most probable scenario” is that the driver had some kind of medical problem while behind the wheel.

Local officials told French media that weather conditions were clear when the bus left the road, and no ice or snow on the section where the bus vehicle crashed. They said all the children were wearing seat belts.

A photo from the town published online showed the bus leaning off the slope of a ravine in a tangle of dried branches with its windshield shattered.