(WBIR - Knoxville) People enjoying the great outdoors have recently encountered some threatening behavior at a popular spot in South Knoxville.

Mountain bikers on the trails at Forks of the River Wildlife Management Area in South Knoxville were sabotaged by strategically placed roofing nails. Aside from several blown tires, nobody was injured.

Running coach Mary Cates told 10News she has also recently run into trouble on the trails that resulted in what she believes is a clear attempt to threaten runners.

"I was running the trail and came up on two guys. They had dogs and shotguns. None of us said a word. We were both clearly shocked to see each other. I just ran past them, then came back and tied my ribbons. They saw me tie my ribbons. Then I just went on my way."

Cates was tying green and pink ribbons to trees to mark a training course for runners preparing for this weekend's "Stalk Your Quarry Trail Race" hosted by the Knoxville Track Club.

Shortly after Cates ran into the strangers on the trail, the ribbons were accompanied by some threatening ornaments.

"There were shotgun shells attached to the ribbons. This [shell] was just placed right inside that ribbon," said Cates. "I don't know. It feels threatening. Maybe that's silly, but it's something we sort of felt like was a 'message' to us."

With the advent of the Urban Wilderness in South Knoxville, trails have expanded into new territory traditionally occupied by hunters. Cates believes that expansion may be the source of tension.

"I understand people use this land to hunt at certain times of year. I completely understand and respect that. But it is not hunting season right now, so if they were hunting it was illegal," she said.

The Tennessee Wildlife Resource Agency (TWRA) owns the Forks of the River WMA and is responsible for law enforcement on the property. TWRA said it has been made aware of the issues and is investigating.

In the meantime, Cates said she wants to make the public aware to prevent injuries to people or pets on the trails. Cates said she still plans to happily use the trails, but will be sure to follow normal safety advice such as always running or riding with a partner and keeping an eye on the ground for potential hazards.

Most of all, Cates said she does not want an incident full of nails and shotgun shells to prevent people from enjoying the wilderness. She said the sight of ammo casings tied to ribbons did cause a bit of shell shock for her runners.

"Those individuals who saw that shotgun shell, they were pretty scared. For the safety of people who come out here to ride and bike, something has got to stop. I'd hate for somebody sending a threat to ruin this for people who just want to love the outdoors," said Cates. "Knoxville is full of gorgeous places to trail run and people don't even know these places are here. It is so wonderful out here and I just want everyone to be able to enjoy it and be safe."