MECHANIC FALLS, Maine — A mechanical problem caused a Jeep towing a wagon full of passengers to careen down a steep hill and slam into a tree during a Halloween-themed hayride in the woods, killing a teenager and injuring more than 20 others, authorities said.

Investigators were inspecting the Jeep to try to determine exactly what kept it from stopping on the hill, and state police were calculating the passengers’ weight to determine if the hay wagon was overloaded and whether that contributed to the mechanical problem, Sgt. Joel Davis of the state fire marshal’s office told reporters.

The Saturday night crash during the Gauntlet Haunted Night Ride “threw everyone off the trailer and into each other and into trees,” Davis said.

Seventeen-year-old Cassidy Charette of Oakland, Maine died from head injuries, state police spokesman Steve McCausland said. She was among a group of friends from Messalonskee High School who visit Harvest Hills Farm every fall, authorities said.

Among them was 16-year-old Connor Garland of Belgrade, who suffered multiple fractures and was being treated at Boston Children’s Hospital,

Kathy Mathieu of Oakland, whose son Zachary is a close friend of Garland’s, said Charette and Garland attended the school’s homecoming celebration together last week. She said the community is planning a fundraiser for Charette’s family.

Charette was a member of the school’s girls’ soccer team. Garland is a member of the Central Maine Owls, a 15-and-under baseball team that won the state championship this year.

Messalonskee Principal Jon Moody sent a statement to the school community saying grief counselors would be available.

The hay wagon was being pulled by a 1979 Jeep when it crashed, according to the fire marshal’s office. The driver, identified by police as David Brown, 54, of South Paris, was among those injured. Authorities said he underwent surgery late Saturday. Brown is an experienced trucker who has a commercial driver’s license, according to a spokesman for the farm.