A bicyclist riding in the Trek Across Maine fundraiser was killed Friday morning when he apparently fell and was run over by the rear wheels of a tractor-trailer in Hanover, just a few miles from the ride’s starting point.

David LeClair, 23, of Watertown, Mass., was killed almost instantly about 8:45 a.m. as the truck passed him along U.S. Route 2, according to Maine State Police.

As trek cyclists arrived at their overnight campsite at the University of Maine at Farmington’s Prescott Field Friday afternoon and news spread of the accident, cyclists said the the route was well-marked with warnings for drivers. Riders said law enforcement personnel, volunteers and clear signs are positioned along the highway, warning drivers to watch for people on bikes.

U.S. 2 is the state’s major east-west highway, heavily traveled by both cars and trucks.

First-time participant Terri Brown, 23, of Brunswick, said organizers made an enormous effort to direct traffic around the cyclists.

“There are so many volunteers and cops out there. You can’t not know to drive slow and watch out, because there are like thousands of bikers out there,” she said.

John Harris, 73, of South Bristol, said in the 10 years he’s participated in the trek, which draws about 2,000 cyclists each year, drivers have always been willing to share the road and brake for riders. He said there are always people guiding vehicles around the participants at all major intersections.

“I’ve never felt threatened by a car,” he said.

Matt O’Neill, 59, of Buxton, said he heard about the accident from other cyclists when they took their first rest stop shortly after it occurred.

He and his wife, Carmen O’Neill, said they have participated in the trek for 13 years and said cars mostly keep a safe distance from the cyclists.

“Some drivers get impatient, but they are mostly respectful,” he said.

Officers said both the truck and the cyclists were headed east. LeClair may have fallen off the bike before he was killed, although it’s not yet clear what might have caused him to fall. His death is the first bicycle fatality of 2013 in the state, according to law enforcement.

Police stopped a truck in Rumford they believed was involved, said State Police Lt. Walter Grzyb.

The name of the driver of the tractor-trailer, which is owned by Transport Beauregard Inc. based in Quebec, has not been released. A man who climbed down from the driver’s cab declined to be interviewed. Serge Beauregard, who operates the Canadian transport company, did not return a call requesting information about the driver’s experience, safety record or length of service.

“Part of this investigation is to look closely at his driving log and hours of operation, making sure he’s in compliance and making sure his truck is safe,” Grzyb said. “We believe we have the truck. We’re going to have to do some additional investigation to confirm this is the truck.”

Investigators are examining whether the force of wind generated by the passing truck upset LeClair’s bicycle, which was apparently undamaged. The speed limit in the area is 40 miles per hour.

Truck drivers must comply with federal rules that govern how many hours they are allowed to drive without sleep. Grzyb said the driver told police he was not aware that an accident took place, which Grzyb said is common for large trucks involved in such accidents.

Meanwhile, police are asking for riders’ help in gathering any video footage that may have captured the accident.

LeClair was riding as part of a 140-member team of employees of athenahealth. A 2011 graduate of Bentley College in South Waltham, Mass., LeClair was originally from Naples, Fla., and worked at athenahealth for two years. He helped doctors and their practices better interact and communicate with insurance companies, said Holly Spring, an athenahealth spokeswoman. His family could not be located for comment.

Spring said the company has counselors on hand at its Watertown, Mass. office to help his co-workers process the loss, and are sending counselors to Maine to assist the remaining company riders.

“We’re all pretty sad here,” Spring said. “We’re going to do everything we can to celebrate David’s life and support our employees.”

Spring said athenahealth operates an office in Belfast, employing 600 people. An additional 1,000 work at the Watertown, headquarters.

Bob Moulton, whose home is across from the scene of the accident, said his adult son was watching the group of about 12 riders pass the house, saw the truck close the riders, but looked away a few seconds before the accident.

“(My son) turned away and that’s when the screaming started,” Moulton said Friday. He ran outside to find a chaotic scene at the road’s edge, where LeClair’s body lay on the white line separating the breakdown lane and the travel lane.

Grzyb said witnesses said LeClair was riding abreast with another cyclist, with LeClair on the left, closest to the travel lane. Police are still trying to determine if oncoming traffic may have affected how close the truck came to the shoulder.

Nancy Grant, executive director of the Bicycle Coalition of Maine, who is also participating in the trek, said organizers mark the route and intersections, station volunteers along the way and provide signs in areas that require extra attention. Grant said she was one of a team of 30 riders who are designated to act as safety patrol.

“The trek does a really good job trying to ensure everyone’s safe,” Grant said.

Jeff Seyler, CEO of the American Lung Association of the Northeast, sponsors of the annual trek, issued a statement saying the event will continue.

The 180-mile Trek Across Maine was in its first day, with riders leaving from Sunday River Resort bound for an overnight layover at the University of Maine at Farmington, a 53-mile ride. The event, which concludes on Sunday in Belfast, benefits the American Lung Association, drawing an estimated 2,000 riders each year. This year the charity raised $1.35 million.

Morning Sentinel Staff Writer Kaitlin Schroeder and Press Herald Staff Writer David Hench contributed to this story.