Kentucky Derby winner Medina Spirit, who provided trainer Bob Baffert with a record seventh win in the race, has failed a drugs test.
Baffert said he was “shocked” after the horse tested above the limit for betamethasone and insisted he had never administered the substance.
“I got the biggest gut punch in racing for something I didn’t do,” he said.
Medina Spirit, ridden by John Velazquez, won America’s most famous race at Churchill Downs on 1 May.
Baffert, 68, was the first trainer to win the US Triple Crown for 37 years when American Pharoah completed the treble of Kentucky Derby, Preakness Stakes and Belmont Stakes in 2015.
The Hall of Fame trainer repeated the hat-trick with Justify three years later.
But on Sunday, he told reporters that Medina Spirit had tested positive for 21 picograms of the anti-inflammatory drug betamethasone, above Kentucky racing’s threshold of ten picograms per millilitre.
“It’s such an injustice to the horse. I don’t feel embarrassed, I feel like I was robbed,” said the California-based trainer.
“But I’m going to fight it tooth and nail, because I owe it to the horse; I owe it to the owner and I owe it to our industry.
“There are problems in racing, but it’s not Bob Baffert. I don’t believe in conspiracy theories, but why is it happening to me?”
Baffert said Medina Spirit has not yet been officially disqualified from the Kentucky Derby, although that still could happen after other tests and processes are completed.
“This shouldn’t have happened,” Baffert said. “There’s a problem somewhere. It didn’t come from us.”
Churchill Downs has suspended Baffert from having entries for races.
“If the findings are upheld, Medina Spirit’s results in the Kentucky Derby will be invalidated and Mandaloun will be declared winner,” it said in a statement.
“Failure to comply with the rules and medication protocols jeopardises the safety of the horses and jockeys, the integrity of our sport and the reputation of the Kentucky Derby and all who participate. Churchill Downs will not tolerate it.”
The trainer last year pledged “to do better” after a number of positive tests were produced from his horses.
Last month he was successful in overturning a 15-day ban and disqualification of two of his horses – Gamine and Charlatan – after they returned samples containing the illicit raceday medication lidocaine, which was attributed to cross-contamination from a pain-relief patch worn by the trainer’s assistant.
Baffert received a $10,000 fine instead.