Dubai World Cup winner Electrocutionist died of an apparent heart attack Saturday, several days after an abnormality was detected in the 5-year-old horse’s heart.

Dubai-based Godolphin Racing Inc. owned Electrocutionist, considered a top contender for the upcoming Breeders’ Cup Classic Powered by Dodge.

“He appeared to be responding well to treatment, but during the night he suffered what seems to have been a fatal heart attack,” Godolphin racing manager Simon Crisford said. “It is terribly sad. He was a great horse for Godolphin and a real stable favorite. He will be sorely missed by all of us.”

The Kentucky-bred son on Red Ransom won the $6 million Dubai World Cup (UAE-)–the world’s richest horse race–on March 25, and finished second in his last start, the King George VI And Queen Elizabeth Diamond Stakes (Eng-I) at Ascot on July 29. Electrocutionist also finished second behind Ouija Board in the Prince of Wales’ Stakes (Eng-I) at Ascot eight days earlier