Just over halfway through the 2011 Kielder Marathon in Northumberland, Steve Cairns was in third place and out on his own. As he passed the 14-mile mark, he could see the two leading runners, Ricky Lightfoot and Marcus Scotney, a few minutes ahead on the trail. Behind him, just passing the 13-mile point, were the chasing pack. Cairns knew he had little chance of catching the front-runners. He was equally confident that, with a six-minute gap to close, the rest of the field had little chance of catching him. “I’m just going to enjoy this,” he decided.

Cairns held on to his position and finished the race comfortably. As he crossed the finish line, he heard his result called over the PA. “I’m thinking, did I hear that right?” he says. “Fourth?” He asked a marshal to point out who was third. Cairns recognised the man immediately. It was Rob Sloan, the winner of a 10k race held the day before. There was no mistaking him: Sloan had a mohican haircut and distinctive tattoos. Cairns had exchanged nods with him on the start line; this was the first time he’d seen him since. Cairns placed a hand on Sloan’s shoulder. “When did you pass me?” he demanded. “Out on the course,” Sloan shot back.

Sloan didn’t attend the medal ceremony. In the hours after the race, troubling details about his performance began to emerge. No other runners could recall him passing them on the trail. Photographs suggested he was missing from the race with a few miles to go. Then there was the most damning evidence of all. Witnesses clearly recalled seeing Sloan during the last few miles of the race – just not on the course. Sloan had taken a bus.

Race organiser Steve Cram, the Olympic 1,500m runner, called Sloan. “He absolutely denied it initially,” says Cram. “Then he eventually owned up to it.” The BBC reported the story and, soon enough, the tale of the marathon bus cheat had been picked up all over the world. Despite the evidence against him, Sloan retracted his confession. Cram banned Sloan from his races. UK Athletics followed suit and Sloan disappeared from the running scene. Meanwhile, Cairns was awarded his third place medal. “It was vindication,” says Cairns. “But what I couldn’t understand was: why had he done it?”

Facebook Twitter Pinterest Finished: Rosie Ruiz is helped by Boston police after winning the women’s division of the Boston Marathon in 1980 in a new record time. A few days later it was shown that she had only run the last mile or so of the course and was disqualified. Photograph: AP

For a sport with few material rewards, marathon running has produced some illustrious cheats. In 1980, the Boston Marathon was won by Rosie Ruiz, who set a women’s course record. Suspicions were raised by Ruiz’s unflustered appearance at the finish line and witnesses later reported seeing her joining the course with less than a mile to go. Kip Litton, a dentist from Michigan, allegedly cheated numerous times in a bid to run marathons in every state – and claimed first place in a race of his own invention. At the 1999 Comrades 55-mile ultra-marathon in South Africa, two brothers claimed ninth position after running the race in relay, swapping clothes in toilets along the route. They were only caught when photos emerged of the two men wearing watches on opposite wrists at different stages of the course.

Setting aside doping, marathon cheats can be divided into two main categories. There are the bib mules: runners who compete under another competitor’s race number, typically in order to record a qualifying time for another prestigious race. (Bib bandits – runners who forge race numbers to secure entry to events – are a related but distinct category.) Then there are the course cutters, who engage in that most rudimentary cheating tactic: jumping a barrier or ducking under some tape to skip a section of the course.

Nowadays, after almost every major race, a handful of competitors is exposed as cheats. (In some cases, more than a handful: thousands of runners were disqualified after this year’s Mexico City Marathon.) It’s difficult to know if the problem is on the rise or if there are simply more offenders being caught. As technology has improved, marathon cheating has become more of a high-risk endeavour. Competitors are now kitted out with electronic chips that register runners’ progress as they pass over timing mats installed around the course. This provides more accurate times – but it also gives organisers, and anyone who cares to look, a wealth of data to examine for suspicious results. Several nights a week, Derek Murphy settles down in front of the TV at his home in Cincinnati, opens up his spreadsheets and gets to work. By day, Murphy is a business analyst, but he’s become better known as the man behind Marathon Investigation, a blog that for the past three years has been relentlessly exposing cheaters in the marathon world.

Murphy sees many different kinds of cheating. His specialism is in outing runners who fraudulently obtain qualifying times for the Boston Marathon. Sometimes it’s for material gain. Other times it’s for bragging rights on social media. More often that not, says Murphy, the cheating isn’t premeditated. “A lot more of it is where it spirals away from them,” he says: a snap decision followed by lies that get out of control.

Not all runners approve of Murphy’s approach, seeing him as something of a vigilante, but Murphy is unperturbed by such criticism. “Of course there are some people who dislike what I do,” he says. “People say that I’m shaming runners and I should focus on the positive. But I think it’s important to hold people accountable.”

Facebook Twitter Pinterest He’s got your number: Derek Murphy looks at a graph of runner start and finish times as he works at his Marathon Investigations business at his home in Ohio. Photograph: Tom Uhlman/AP/Toronto Star

Any characterisation of Murphy as a lone vigilante also ignores an important fact: he has an army of collaborators. “I’m being tipped off by friends or former friends,” he says. “They’re saying, ‘I know this person, they couldn’t have done this, can you look at this result?’” Murphy is simply the public face of a wider movement determined to enforce standards of fair play.

On 23 April this year a thread was posted to the Runner’s World forum: “London Marathon Cheaters – let’s do this”. It was the day after the race and forum members focused on identifying negative splits – indicating the second half of the race had been completed faster than the first. Significant negative splits, combined with missed timing mats, often indicate a runner who has cut the course. Rory O’Connor was among the first to be named: an Irish runner in the 60-64 age category who appeared to have run nearly nine miles in 15 minutes. He later insisted that “he ran every step”. Plenty of other runners exhibited similarly unbelievable times.

Several runners named in the thread told versions of the same story: they had intended to run the full distance until they became tired or injured. Abigail Willmitt, the secretary of Liverpool Running Club, pulled out of the race just after halfway and says she was taken across a barrier to be checked over by medical volunteers. Shortly afterwards, a friend passed by who was struggling. Willmitt says she was given permission by a marshal to accompany her friend from the 22-mile mark. She crossed the finish line but never claimed a medal. Willmitt struggles to understand why she was named and shamed online. “I’ve never once said that I completed the full distance,” she says.

Chris Walker from Dewsbury attracted particular criticism after forum users reported seeing social media posts in which he claimed to have completed the race. Walker declined to be interviewed but said via email that a long-term illness forced him to drop out of the race. He subsequently made his way to the finish line where he claimed a medal and posed for a finisher’s photo. Walker, who was running to raise money for charity, said he was afraid of letting down sponsors and supporters.

None of this year’s course cutters, however, attracted the level of notoriety achieved by Jason Scotland-Williams at the London Marathon in 2014. Scotland-Williams ran the first half of the race in two hours and seven minutes – a respectable if unremarkable time. But the previously unknown 34-year-old then kicked up several gears, crossing the finish line another hour and one minute later, completing the second half of the race faster than Mo Farah – and not far off half-marathon world record pace.

Sceptics examined his split times and discovered Scotland-Williams had missed three timing mats in the second half of the race. The Sun, in an article headlined Con Your Marks, alleged he had jumped a barrier and cut the course. Subsequent articles dubbed Scotland-Williams the “Faux Farah”. He claims he was bombarded with abuse online and that strangers visited his house to admonish him in person. Throughout it all, he denied that anything but hard work had contributed to his incredible time. As he recalls, at one point the head of the London Marathon called and urged him to confess. He refused.

Four years on, asked to look back on his performance, Scotland-Williams begins by sticking to his guns. “I’m an achiever, I’m a competitive person, I like to be the best at what I do,” he says. “That’s just how I am. If other people don’t have that drive or that thirst for success or that hunger, I can’t help that.” Nothing is impossible, he says, and the anger he provoked was unwarranted. “I broke no laws, I hurt nobody. Nobody lost their life. All I did was move my legs.”

But at quite such an impressive pace? Is it possible he made a regrettable decision that spiralled out of control? Before answering that question, there are some things people need to understand, says Scotland-Williams. Running numerous races for charity had taken its toll and, by 2014, his knees had begun to fail him. “I’m not a person to quit,” he says. “Even if I have to crawl across the finish line, I’m going to cross the finish line.” With that context established, he says: “Was a rash decision made? Did I make a mistake? Possibly. At the end of the day, I achieved my goal. I crossed the finish line.”

But did he cut the course? He declines to elaborate further. “Was a decision made which probably proved to be a bad one? Do I regret said decision? To a degree,” he says. “At the same time, that doesn’t take away from my previous accomplishments, doesn’t take away from what I’ve achieved in terms of fundraising, and certainly doesn’t take away from who I am as a person. Do I deserve the backlash I got? That’s a matter of different people’s opinions.”

Liam Convery was among those discussing suspect results on the Runner’s World forum after this year’s London Marathon. At 47, he has completed the race seven times. He explains that some runners’ anger stems from the scarcity of places at prestigious races. “It is a privilege,” he says. “For someone to take a place that I would have treasured and treated it that way seems morally very dubious.” But there’s more to it than that. For Convery, any marathon runner who takes a shortcut to the finish line is doing a disservice to the sport. “The achievement is in overcoming the adversity,” he says. “To simply take the easy way out: not train, turn up, cut a corner, take the medal, take the kudos for doing it? I’m sorry, I just don’t have a lot of respect for those people.”

There is some consensus among the running community that, at the top level, cheaters must be identified. There, anyone taking a short cut has the potential to deny others their rightful rewards. But how should cheating be dealt with among the thousands of amateurs who run marathons, for whom races represent a personal battle of determination and will? To what extent is victory diminished by the dishonesty of others? Is cheating at this level a matter of personal responsibility?

Hugh Brasher, event director at the London Marathon, declined to be interviewed for this article. In a statement, he said: “We have processes in place which identify the very small number of runners with anomalies in their results and these runners are contacted by our results team to request an explanation. If no adequate explanation is received, their results are removed from the system and the runner is asked to return their finisher’s medal.”

At the Brighton Marathon, race director Tom Naylor says considerable effort is made to ensure any cheats are identified and disqualified. “However, can I say hand on heart that every single person that’s on our finisher list has not cheated or has not cut a corner? No.” From a participant’s point of view, if a fellow runner cut the course, “I’d be disappointed for that person, but I still finished the race,” says Naylor. “And for the rest of my time I know that I’m a marathon runner and I can be proud of myself and I can be proud of the people I ran with the whole way.”

Derek Murphy has changed tactics over time. In the early days, he was prepared to write about anyone. But with increased reach has come greater responsibility. He has taken down some of his early posts concerning amateur cheaters. Now he looks for repeat offenders, or runners who have secured tangible benefits from their dishonesty. Before posting to his blog, he asks himself: “What did they really gain and do they really deserve the attention?”

Murphy is keen to refute any suggestions that his work paints running in a negative light. His motive, he says, is to ensure race places and rewards go to those who deserve them. While he enjoys identifying suspect results, he gains little pleasure from bringing shame upon offenders. “The part I hate is writing the articles and having to contact and sometimes confront people,” he says. “It’s much more rewarding to write about the positives.”

Next weekend, more than 50,000 people will line up to run the New York Marathon. Each will emerge with their own story: of overcoming mental and physical hurdles just to make it to the start line, of summoning previously untapped reserves to run a personal best, of races run in memory of loved ones or in support of meaningful causes. Most of those stories will never be widely told – but will be no less significant for that. And for those who reach the finish line by dishonest means? Murphy, and a legion of other amateur race investigators, will be watching.