PORTER – A bicyclist riding with his family from Moultonborough, N.H., to Portland was killed instantly Monday morning when he was struck by the rear wheels of a tractor-trailer truck on Route 25.

The accident occurred about 1.8 miles from the New Hampshire border, according to Adam Fillebrown, the state trooper who responded to the scene.

He said bicyclist John Lacaillade II, 38, of Meredith, N.H., was traveling eastbound at a place in the road where “the pavement drops right off,” and the shoulder becomes very narrow, Fillebrown said.

“They got to this spot” in the road, when “the tractor-trailer met two other (oncoming) vehicles,” and Lacaillade “got pulled from or fell off the bike,” Fillebrown said. “He fell underneath the trailer and the tire hit him.”

Lacaillade was bicycling with other members of his family, according to a statement from the Maine Department of Public Safety. He joined the group in Moultonborough for the final leg of their trip, which was to bring them to Portland by the end of the day, according to the statement. Some members of the group had bicycled from California, the statement said.

The accident occurred just before 9 a.m., he said.

An investigation is under way to determine how Lacaillade lost control of his bicycle, if he struck the tractor-trailer before falling from the bike, and the speeds of the vehicles.

Fillebrown did not expect charges to be brought against the driver of the tractor-trailer, Renald Morin, 30, of Quebec. Morin works for the trucking company Loignon Champ-Carr Inc., based in Quebec, and was hauling wood chips, according to the statement. He was not injured.

Seven state troopers and an Oxford County sheriff’s deputy investigated the accident, which forced a six-hour closure of a section of the road, according to the statement.