Tim Fishback was riding the new Little Scraggy Trail in the Buffalo Creek network last weekend when his buddy, Nick Kostecki, got two flat tires in a relatively smooth section. When Fishback returned to help his pal, he too flatted.

They poked around the trail and found a nail sticking out of the dirt. They tried to pry it up but it was mounted in a concrete brick. More riders on the trail that day reported flats. Across a mile span, Fishback and his friend found three 2-pound bricks that had been formed around 3-inch nails and buried in the middle of the singletrack.

“I’ve never seen anything like it. Someone manufactured these things. It took some effort to dig those holes and put those in there and they were spaced out a good bit. Somebody was trying to do some real harm and they were targeting bikers,” said Fishback, who regularly rides the Buffalo Creek area, where mountain bikers have spent two decades working with the U.S. Forest Service to develop and build one of the state’s richest webs of flowing singletrack. “That’s the craziest thing. Mountain bikers built that trail. I rarely see anyone but mountain bikers on that trail.”

Mountain bikers volunteered more than 1,100 hours of labor to build the Little Scraggy Loop, which opened in 2014 with 9 miles of carefully sculpted singletrack winding through the Buffalo Creek Recreation area in the Pike National Forest. A 20-year partnership between the Forest Service and the Front Range mountain biking community has transformed t he Buffalo Creek area from a scorched Jefferson County burn zone into one of Colorado’s top destinations for the knobby-tired tribe.

The Forest Service is investigating and declined to comment.

It’s not the first time Colorado mountain bikers have found their trails sabotaged. A biker in June 2014 found two boards spiked with long nails along a trail in the heavily-traveled Prince Creek network near Carbondale. The Bureau of Land Management launched an investigation. They never found the person responsible but area bikers suspected it was long-term campers who were occupying undeveloped camping sites in the area. No one has reported any other similar incidents since.

“Unfortunately those acts are not only sabotaging the trail system but they are putting people at harm and maybe causing some unintended consequences,” said Todd Fugate, a board member of the Roaring Fork Mountain Bike Association. “Luckily the guy who saw the boards was on the ascent and going slow. If he had been coming down fast, he wouldn’t have been able to stop and it would have resulted in injury.”

Similar cases of trail sabotage have attracted attention in California, the Pacific Northwest and British Columbia:

• A 64-year-old woman was convicted of criminal mischief and sentenced to probation and community service in January after wildlife cameras caught her draggling tree limbs across trails in North Vancouver. The avid hiker would spend hours a day setting traps for cyclists on trails built by mountain bikers.

• Three years ago a 57-year-old psychiatrist was sentenced to jail for booby-trapping Oregon mountain biking trails by stringing rope across trails and hiding nails in vegetation spread across singletrack.

• In 2011, an anti-mountain biking zealot renowned for his vitriolic diatribes against bikes was sentenced to prison for attacking mountain bikers near his Oakland home.

• Police arrested a man in 2008 for digging and then concealing more than 50 deep holes in a mountain bike trail in southern California.

Mark Eller, the communications director for the Boulder-based International Mountain Bicycling Association, really wants to call these isolated incidents that involved crazy people, and certainly not a national trend.

“It happens enough now that I would say there is a worrisome aspect where people can see these as passive attacks on different user groups,” Eller said. “The factual way of looking at it is that this happens a few times a year now.”

Conflicts between mountain bikers and hikers are as old as knobby tires. Hikers can have a peaceful stroll disrupted by speeding cyclists. Bikers can have their joy tainted by an angry walker. People who have been on a trail 1,000 times can develop a sense of ownership of that public area and maybe grow angry with newcomers. The ire has always simmered on the singletrack. Now it’s increasingly common to see that anger turn into something violent.

“It’s concerning that it seems to be almost a copy-cat element here,” Eller said. “These things are publicized really well and you hope that makes it all go away but you never really shake it.”

Keith Clarke, who co-founded the Front Range Mountain Bike Patrol that has worked with the Forest Service to develop trails in Buffalo Creek said his team has seen flags removed from trail re-routes during building phases and the occasional graffiti on trailhead maps.

“But we’ve never seen anything like this before,” Clarke said. “We are obviously very concerned.”

Bikers have labored over the past three decades to paint themselves in a better light. Thirty years ago they were the unwelcome newcomers to the trail, riding fast in places where people had long ambled. Bikers donate thousands of hours to train maintenance, like at Jefferson County’s Apex Trail, where a group of dedicated riders have rebuilt miles of singletrack damaged in the 2013 floods. Groups like IMBA work to educate riders about trail etiquette, which essentially involves yielding to every other user on the trail.

“It doesn’t help that there are more and more people on the trail,” said Fishback, 39. “But maybe if you are unhappy about something, you can complain to someone. There are avenues to be upset about things. To try to hurt people, there is something wrong with that thinking.”

Jason Blevins: 303-954-1374, jblevins@denverpost.com or @jasonblevins