Andy Serling is a handicapper for the New York Racing Association. His job is to tell Fox Sports viewers which horse he thinks is the likely winner. And before one race at Saratoga Race Course this month, Mr. Serling nailed it.

He selected a filly named M B’s Melanie Cares at odds of 13 to 1. She had not run in seven months. Her workouts were slow. She was trained by a horseman he had never heard of.

On paper, there was not much to recommend M B’s Melanie Cares. Except Mr. Serling had noticed that in an earlier race her trainer, Ernesto Ochoa, had a horse named Classic Rock who won by nearly nine lengths at odds of 9 to 1 after a long layoff with slow workouts.

Mr. Serling suggested to his viewers that a “betting coup” was underway. It is a practice as old as horse racing itself, where owners and trainers make the horse appear slow heading into a race to keep the odds high and cash in big after winning.

Mr. Serling told his audience that he was putting a few bucks of his own on M B’s Melanie Cares, who he predicted could pay out $35 on a $2 bet.

M B’s Melanie Cares won by almost four lengths and paid $27.70 for a $2 bet — nearly what Mr. Serling had predicted and a windfall that set the upstate racetrack stands buzzing.

That buzz intensified when horseplayers noticed a similar set of circumstances — on the same day, Aug. 9 — at Monmouth Park on the New Jersey Shore, where another trainer, Angel Quiroz, won back-to-back races with a pair of horses with similarly underwhelming résumés.

The four winning horses were trained by Mr. Quiroz at Fair Hill Training Center in Elkton, Md., and the two Saratoga winners were sold to Mr. Ochoa before their blowout victories. Unlike racetracks where every horse is timed by independent clockers, Fair Hill has trainers do the timing themselves and report the times to the track where they intend the horse to compete. This system offers the opportunity to “darken” a horse’s form with slower times to make the thoroughbred look less appealing to bettors.

On social media, the horsing world was aflutter about how the trainers got “one over,” in racetrack parlance, in mostly admiring terms.

But the racing industry’s regulator was not amused and has opened an investigation into the two trainers and the training center.

Lisa Lazarus, chief executive of the Horseracing Integrity and Safety Authority, a quasi-public federal regulator, said that the horses had undergone extensive drug and DNA testing and that the sports’ tote system was being examined for suspicious betting. She said none of the results were back.

“HISA takes this situation very seriously, and if any violations are uncovered there will be serious consequences,” Ms. Lazarus said.

But darkening a horse’s practice form with slow workouts is hard to prove, and suspected instances have rarely been punished. Short of finding evidence that the races were fixed or the horses had been given performance-enhancing drugs, it will be difficult for HISA to hand down meaningful penalties to the trainers.

Mr. Quiroz and Mr. Ochoa said they had done nothing wrong other than having a very good day on the track.

“I have nothing to hide — I am clean,” Mr. Quiroz said in an interview, adding that he had reported accurate training times.

Mr. Ochoa said he felt discriminated against as a small-time trainer. “I win two races at the best racetrack in the world and people aren’t happy,” he said.

The intrigue stretched all to way to London, where several bookmaking shops had taken bets on the Saratoga and Monmouth races from an array of customers, many of them American.

The bookmakers suffered significant six-figure losses, according to two people familiar with the investigation who were not authorized to speak publicly about it.

“It was a fairly remarkable attempted gamble, the likes of which we’ve never really seen before,” Paul Binfield of Paddy Power, a major bookmaker, told Britain’s Racing Post. “It’s cost us a few quid, but nothing eye-watering.”

Both Mr. Quiroz and Mr. Ochoa said they knew nothing about bets placed with English bookies.

Betting coups are the stuff of racetrack lore.

Inside information about horses and races is traded frequently, most of it useless. When a big score is made public, it often is celebrated as long as it does not involve doping or other tactics harmful to a thoroughbred.

“I always have a grudging respect for people who pull it off,” Mr. Serling said, echoing a sentiment common among horseplayers.

Mr. Quiroz said he sometimes bets on his horses, but did not that day. He said he and Mr. Ochoa are friends. “Are we not allowed to win races?” Mr. Quiroz said.

The questions around Mr. Quiroz and his horse have now been compounded by another issue. Last Thursday, regulators announced that a horse not involved in the races and trained by Mr. Quiroz had tested positive for albuterol, a beta-agonist and bronchodilator used to treat equine asthma or respiratory distress in horses. It can be repurposed to build lean muscle mass when it is abused, according to regulators.

When asked in a follow-up interview about the drug test results, Mr. Quiroz said that there had been a death in his family and that he was unable to comment.

Also coming under scrutiny is the Fair Hill Training Center. It’s a bucolic setting, with riding trails and dirt, turf and synthetic surfaces. It is home to the Kentucky Derby-winning trainers Graham Motion and Shug McGaughey.

Will Phipps, Fair Hill’s executive director, said Mr. Quiroz’s three-month lease to train horses at the facility ends in September. He said the training’s center board was also investigating.

Mr. Quiroz said he had moved his operation to Fair Hill from Florida two months ago. Over the past 17 years, he has won 178 races for more than $3.3 million. Mr. Ochoa has won 25 races in the last nine years for almost $900,000.

“I wanted more options,” Mr. Quiroz said. “It’s a short drive to racetracks in Maryland, New Jersey and Pennsylvania.”