James Knapp, 64, was found bleeding from his head, arm and leg on the Trail near 21st and Winthrop on the city's near north side around noon. He was hospitalized.
Police said Knapp wasn't sure if he had been robbed.
"He doesn't really know exactly what happened," said IMPD spokeswoman Sgt. Linda Jackson. "He was just kinda maybe ambushed, hit and then knocked down."
Jackson said a juvenile detective was assigned to the case and is looking over video from a crime camera located in the area. The incident raised concerns about the safety of the Monon Trail.
"It's in the back of my mind that something could happen," said Brian Williams, a bicyclist who often rides the Trail. "It's very sad that the Monon is a community property that you can't feel safe on."
Another bicyclist, Jeff Stonebraker, showed Fox59 the mace he bought, because he doesn't feel safe on the Trail.
"I just got it when I was gonna start riding this Trail so I would have something if needed," Stonebraker explained. "I'm not sure I could really break it out and spray and what I would do exactly, but it's something."
According to Jackson, police have been called out to the Monon Trail 33 times so far this year. Of those calls, two were reports of assaults and two others were for strong armed robberies. She believes the Monon Trail is safe, though.
"I don't have anything to indicate that this is nothing but a random incident and we've got our resources deployed out there to see if we can find whose responsible for this," she said.
The security officer told police the teens were wearing t-shirts and black shorts and riding red/black, red/silver and black/yellow 10-speed bikes.
If you have any information about the recent incident, contact police immediately.