A grueling cycling race is somewhat less grueling if your bike is a motorcycle. Understanding this, some cunning cyclists might be turning the sport into NASCAR on two wheels by surreptitiously giving their bikes a motorized boost.

The first confirmed case of mechanical doping surfaced earlier this year when a tiny motor and battery were found inside a Belgian cyclist’s bike, but that involved cyclocross, a comparatively minor branch of the sport. The latest accusations emerged Sunday on Stade 2, a French television network that is also the host broadcaster of the Tour de France. The report suggested that motordoping is also at the highest levels of the sport.

Suggestions that top riders are rigging their bikes have escalated during the past several years. As was the case in the early 1990s with more conventional doping, riders who are the targets of such accusations have dismissed them.

But several current and retired professional riders, including the American Greg LeMond, are among those who have said it is a real problem.

Brian Cookson, the president of the International Cycling Union, has made the search for technology-based cheating a priority.

Suspicions stem from two factors: The technology exists, and there is an ever-growing library of videos that show suspicious performances and actions by both riders and teams.

Anyone can buy systems that hide small motors and batteries inside bikes. Marketed as a way to help older or infirm people keep cycling, most of the systems power the axle that joins the two crank arms of the bike and are outwardly invisible, with on-off switches hidden under handlebar tape. Newer, even smaller motor systems can slip into the rear hub to boost the bike from there.

For its report, Stade 2 positioned a thermal imaging camera along the route of the Strade Bianche, an Italian professional men’s race in March held mostly on unpaved roads and featuring many steep climbs. The rear hub of one bicycle glowed with almost the same vivid orange-yellow thermal imprint of the riders’ legs. Engineers and anti-doping experts interviewed by the TV program said the pattern could be explained only by heat generated by a motor.

The rider was not named by the program and could not be identified from the thermal image.

The program also used the camera at a gran fondo, an amateur and semiprofessional event, in Italy. At least one bike showed a suspicious heat pattern around its cranks.

Corriere della Sera, an Italian newspaper that collaborated with the French program, reported that the thermal camera found signs of motors in seven bikes used in the Strade Bianche as well as at the Coppi and Bartali, another Italian race.

Cycling’s equivalents of the Zapruder film are online videos that show unusual patterns of bike changes that precede or follow exceptional bursts of speed by riders. Other videos analyze riders’ hand movements for signs of switching on motors. Still other online analysts pore over videos of crashes, looking for bikes on which the cranks keep turning after separation from the rider.

Unlike the thermal images, however, the videos have only implied that a motor was present.

In a statement, UCI said it had tested and rejected thermal imaging.

“The UCI has been testing for technological fraud for many years, and with the objective of increasing the efficiency of these tests, we have been trialing new methods of detection over the last year,” the governing body said. “We have looked at thermal imaging, X-ray and ultrasonic testing, but by far the most cost effective, reliable and accurate method has proved to be magnetic resonance testing using software we have created in partnership with a company of specialist developers.”

Its system uses a device attached to a tablet computer that sends out a magnetic field. Bikes that create unusual disruptions of that field are then pulled for physical inspection.

About 2,000 bikes have been scanned this year, although no inspection took place at the Strade Bianche.

The magnetic resonance device was responsible for the world’s first confirmed mechanical doping case. At the under-23 women’s event at the world cyclocross championships in late January, it showed a suspicious pattern in a bike belonging to Femke Van den Driessche. A subsequent physical inspection found a motor and battery.

After initially claiming that the bike no longer belonged to her, Van de Driessche, 19, abandoned her defense just before a disciplinary hearing opened and quit cycling.

Earlier in the decade, it was widely believed that motorized cheaters used a system from Austria now called Vivax Assist. But because that device is comparatively heavy and noisy, the general assumption now is that cyclists turn to Stefano Varjas, a Hungarian who demonstrated his products to Stade 2.

In an interview Monday, Varjas said that his crank-assist devices can produce more than 250 watts, the amount of power a professional rider might typically average during a four-hour race. The smaller hub-assist motors, which he makes only for custom orders, typically produce only about 25 watts, he said, and require the rider to be able to maintain a high pedaling rate as is the case with all professionals. Even a 25-watt boost would be significant during a professional race.

Varjas said his system was nearly silent and light enough to keep a bike at the cycling union’s minimum weight.

“If you have this system, you can stay with the group but nobody hears it, nobody sees it, nobody knows about it,” he said of the devices, which cost from 10,000 to 25,000 euros (about $11,300 to about $28,200), depending on features.

While Varjas said that some professionals and teams use the motors for training — sometimes as a substitute for pacing at high speeds behind a motorcycle — he said that he did not know if it was also used in races for cheating. But he added that he believed that some kinds of carbon fiber — the material used to make pro bikes — could render it invisible to the cycling union’s new screening devices.

Varjas said that many of his customers who merely rely on the device to keep cycling are reluctant to offer endorsements.

“It’s a very strange market,” Varjas said. “No one will say they have this kind of bike.”