Los Angeles - Justify, the colt that swept to US racing's Triple Crown in 2018, failed a drug test before that year's Kentucky Derby and shouldn't have been eligible to race, The New York Times reported on Wednesday.

The newspaper reported that Justify, trained by Bob Baffert, tested positive for scopolamine, banned as a potential performance-enhancer, after winning the Santa Anita Derby in California on April 7, 2018.

"That meant Justify should not have run in the Derby, if the sport's rules were followed," the Times said.

According to the report, citing documents reviewed by the newspaper, the California Horse Racing Board took more than three weeks to notify Baffert that the horse had failed the test, advising him nine days before the Kentucky Derby.

It was more than a month before the CHRB confirmed the test result, the Times said, and it was four months later - after Justify had won the Kentucky Derby, Preakness and Belmont stakes to become the 13th Triple Crown winner - that the board voted at a closed-door executive session to dismiss the case.

The Times reported that the decision to dismiss rested on the determination that the positive test could have been the result of contaminated food.

The CHRB later changed the penalty for a scopolamine violation to the lesser penalty of a fine and possible suspension.

Rick Baedeker, executive director of the CHRB, told the Times that the board moved cautiously because scopolamine can be found in jimson weed, a wild plant that can inadvertently show up in horse feed.

"There was no way we could have come up with an investigative report prior to the Kentucky Derby," Baedeker told the Times.

"That's impossible. Well, that's not impossible, that would have been careless and reckless for us to tell an investigator what usually takes you two months, you have to get done in five days, eight days. We weren't going to do that."

The report comes as horse racing in the United States is under scrutiny.

Santa Anita, the famed racetrack outside Los Angeles, was shut down briefly this year as officials probed a spate of equine deaths.

Thirty horses died at the course - which will host the Breeders Cup in November - in the space of six months, prompting California Governor Gavin Newsom to give his backing to legislation which would give local authorities greater power to suspended horse racing licenses.

Animal rights activists seized upon the death toll to renew calls for an outright ban of horse racing.