The 5-year-old gelding named Ky. Colonel had five wins in 20 career starts. He last raced at Golden Gate Fields in Northern California.
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The death was also the second in Mandella's barn since December.
Racing was expected to resume Sunday at the park, which is under intense scrutiny prompted by the series of deaths since Dec. 26. A 3-year-old colt name Emtech was euthanized on the track on the second day of horse-racing season for the park.
The deaths have prompted numerous protests. The park announced rule changes including banning race-day medications designed to protect the thoroughbreds.
In response to the spate of horse deaths, Gov. Gavin Newsom signed a law in June allowing the agency overseeing horse racing in California to immediately suspend licenses to protect the health and safety of horses and riders.
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The ongoing controversy is blamed for a significant drop in the track's attendance.
Santa Anita Park is scheduled to host the Breeder's Cup the first week of November, and the fall season concludes Nov. 3.
City News Service contributed to this story.