Tacks were found covering a popular metro bicycle and running trail last weekend.
They were found along the American Discovery Trail and nearby sidewalks on the east side of the Des Moines River near Lutheran Medical Center last Saturday.
Bikers report to KCCI that they picked up, at least, three handfuls of tacks on the trail -- some already embedded in their tires.
Riders have a message for whoever did this: "Don't ever do it again. I mean, it's malicious and you're going to hurt somebody. A flat tire -- your bike wobbles, no brakes -- you will fall over," said rider Dave Freylack.
The bikers said they tried to pick up all the tacks, but may have missed some.
The first group reported finding close to 80 tacks. Another rider took his leaf blower to the area and found another 20 tacks.
They are advising riders to watch carefully when riding through the area.