LOS ANGELES (AFP) - Drug-tainted Kentucky Derby-winner Medina Spirit died suddenly during a workout in California on Monday (Dec 6), officials said.

The California Horse Racing Board said in a statement that the three-year-old colt, who is trained by legendary US trainer Bob Baffert, collapsed near the end of a morning workout at Santa Anita Park.

"(Medina Spirit) was just completing a workout on the main track at Santa Anita this morning when he collapsed near the finish line. He died immediately," the board statement said, describing it as a "sudden death".

Medina Spirit, who failed a drugs test after winning the Kentucky Derby earlier this year, will now undergo a postmortem to determine the cause of death.

"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 CHRB said.

"Cause of death cannot be determined until the necropsy and toxicology tests have been completed."

The Thoroughbred Daily News racing website, meanwhile, quoted Medina Spirit's owner Amr Zedan as saying the horse had died from a heart attack.

"He had a heart attack," Zedan told the site. "It was quick and he didn't suffer. It's unfortunate. In a moment like this there is not much that we can do.

"All I can say is that he gave us the ride of our lives and brought everyone together. We are mourning this loss, Bob (Baffert), myself, our team and (jockey) Johnny (Velazquez), as well. We are all very sad."

Medina Spirit's victory at the Kentucky Derby on May 1 was called into question after the horse tested positive for traces of betamethasone, an anti-inflammatory steroid medication not allowed to be used within 14 days of competition.

The Kentucky Horse Racing Commission is investigating the matter while Churchill Downs issued a two-year ban on trainer Baffert, who has denied all wrongdoing.

Medina Spirit was the fifth horse trained by Baffert in the past 12 months to fail a drugs test.

Baffert argued the betamethasone found in Medina Spirit's system was the result of a legal application of a topical ointment, rather than an injection.