HANOVER, Maine — A bicyclist from Massachusetts participating in the Trek Across Maine was killed Friday morning when he was struck by the back end of a tractor-trailer while riding on Route 2.

David LeClair, 23, of Watertown, Mass., who was a part of the approximately 140-member athenahealth cycling team, died at the scene about 8:45 a.m., said Stephen McCausland, spokesman for the Maine Department of Public Safety.

“LeClair apparently was struck by the rear portion of the tractor trailer as it passed the bicyclist and was killed instantly,” McCausland said. “Friends he was riding with stopped to render assistance and called police with a description of the truck which did not stop.”

A truck of a similar description was stopped by police a short time later in Rumford, which is about 9 miles from the scene.

“The truck driver, we have interviewed,” McCausland said Friday afternoon. “He did not notice anything. He said he passed hundreds of bikes.”

Investigators are now focusing their attention on the truck, which was hauling corn, he said. McCausland could not confirm reports that the truck was from Quebec.

“We have seized the truck,” McCausland said. “We’re not releasing the name of the driver because we’re not absolutely certain that this is the vehicle.”

Forensic scientists are in the process of examining the truck’s trailer for trace evidence linking it to LeClair, “to be certain that it was the vehicle involved,” he said.

The Trek Across Maine is a three-day, 180-mile bicycle ride that started at Sunday River ski resort in Newry and ends in Belfast. More than 2,000 cyclists are participating, according to the Sun Journal of Lewiston. The event, an annual fundraiser for the American Lung Association, is continuing.

LeClair graduated magna cum laude from Bentley University of Waltham, Mass., in 2011, according to his Linkedin page. He started working for athenahealth in December 2011 as a r ules and f ormat m anagement a ssociate, the site says.

“As someone who lives with environmental-induced asthma, this is personal for me,” LeClair posted on his Trek Across Maine profile page.

The fatal collision occurred about 10 miles into the event, which last year drew 2,100 riders from all over the United States.

“It is with deepest sadness that the American Lung Association of the Northeast reports the passing of a member of our Trek family at our annual Trek Across Maine today,” Jeff Seyler, CEO of the American Lung Association of the Northeast, said in a statement. “All of us at the American Lung Association express our profound sadness at this tragic loss of life and offer our deepest condolences to the family. With heavy hearts, the Trek is continuing.”

Brewer resident Andrew Neff, a participant in the trek, said riders were detoured around the accident scene using local side roads.

Neff, a former Bangor Daily News reporter, said he went past as emergency personnel worked to save LeClair’s life while the Massachusetts man’s friends and teammates stood nearby. Neff said people were still in shock hours later.

Grief counseling is being offered at the University of Maine at Farmington, which is the end point for the first leg of the trek, he said. The second day of the trek ends at Colby College in Waterville.

The finale is at the waterfront in Belfast, which is home to an athenahealth office employing approximately 600 people. LeClair, a native of Naples, Fla., worked at the company’s headquarters in Massachusetts.

Asked about safety measures to protect bicyclists, Neff said the trek’s route is marked with little yellow signs along the roadways and bright orange paint is used every half-mile or so to mark the paved shoulder as the bike route.

“They have people at all the intersections, people with flags and cops [to direct riders],” Neff said of the trek organizers. “It’s pretty well supported.”

Maine law stipulates that motorists must give at least 3 feet of clearance when passing bicyclists and that bicyclists have the same rights and responsibilities as motor vehicle operators.

The Bicycle Coalition of Maine has safety teams supporting the trek and a more than 15-person team participating this year, Brian Allenby, communications director for the group, said Friday. Members are shocked and saddened to hear of the fatal accident, he said.

“Our thoughts and prayers are with the riders’ family and friends and all of those riding this weekend,” Allenby said.

Trek Across Maine organizers have their own specific protocols to ensure rider safety and the coalition is there to support them, he said.

“Ensuring the safety and rights of cyclists is at the core of the Coalition’s mission and we will continue to work with the Trek to provide a positive and safe environment for the remainder of the ride,” Allenby said.

The trek, first held in 1985, is the “largest American Lung Association cycling event in the country,” the group’s website states.