Medina Spirit, who failed a drug test after he won the Kentucky Derby this year, died during a workout Monday morning in California.
The horse collapsed in training at Santa Anita and suffered an apparent heart attack, his trainer, Bob Baffert, said.
Baffert said his "entire barn" was devastated by the 3-year-old colt's death.
"Medina Spirit was a great champion, a member of our family who was loved by all, and we are deeply mourning his loss," Baffert said in a statement. "I will always cherish the proud and personal memories of Medina Spirit and his tremendous spirit. Our most sincere condolences go out to Mr. Amr Zedan and the entire Zedan Racing Stables family."
California's Horse Racing Board said Medina Spirit was just finishing his workout when he collapsed at the finish line. He died instantaneously, the board said.
"All horses that die within facilities regulated by the California Horse Racing Board undergo postmortem (necropsy) examination at a California Animal Health and Food Safety diagnostic laboratory under the auspices of the University of California, Davis," the racing board said.
An official cause of death awaits the results of the examination and toxicology reports.
The horse tested positive for betamethasone after the Kentucky Derby on May 1. Betamethasone is a legal medication that is not permitted on race days.
Baffert denied accusations that he had drugged the horse after a split urine test, saying the drug was in Medina Spirit’s system because of a topical ointment.
The controversy left a stain on the legacies of Baffert and Medina Spirit. Baffert was suspended from the Churchill Downs track for two years in June.
Baffert sued for additional testing to prove that the betamethasone had come from the ointment, Otomax. The horse was not officially disqualified at the time of his death as the investigation was ongoing.
Santa Anita has also been subject to investigation in recent years. California Gov. Gavin Newsom ordered an independent review after 29 horses died at the racetrack in 2018 and 2019. A report published last year by the California Horse Racing Board cleared the track of criminal wrongdoing, saying there was no evidence of animal cruelty or unlawful conduct.
The board did, however, recommend a series of changes after necropsies found that many of the horses who died at Santa Anita had pre-existing issues.
Some of the recommendations included a standardized protocol for when to move races off, additional veterinary examination, digitizing veterinary records for the horses and more frequent testing of track surfaces.
CORRECTION (Dec. 6, 2021, 9:25 p.m. ET): A previous version of this article misspelled the first name of the governor of California. He is Gavin Newsom, not Gavon.