Officials from the Paris to Ancaster bike race are investigating whether the charity event was the target of sabotage.
An uphill section of the course through the community of Harrisburg was blocked Sunday by several large trees that had been cut down and blocked the route.
Organizer John Thorpe says that section of the path was inspected Saturday afternoon and was clear.
But by the time the first group of racers when through the area it was blocked and they had to dismount to climb over the trees and that caused a back up.
Going forward, Thorpe says, organizers will check the course the morning of the race, and he plans to talk to police about what more can be done.