A 3-year-old gelding named Truest Reward was injured while training Thursday morning, according to park officials.
Truest Reward is the 38th horse to die at the track since last December.
The park was supposed to have launched its 83rd winter meeting on Thursday, but rainy weather pushed the opening back to Saturday.
The track was closed for full workouts but was open for jogging and galloping. Truest Reward, a Pennsylvania-bred horse, was jogging or galloping on the six-furlong training track at the time of the injury, which unseated the rider. The horse had previously run in four races, placing second and third in two of them.
Santa Anita and the sport of horse racing have faced increasing pressure from animal-rights activists in connection with the horse deaths, which included 2017 Breeders' Cup Dirt Mile winner Battle of Midway while training at the track on Feb. 23 and Mongolian Groom, who was euthanized after suffering a fatal injury in the Breeders' Cup Classic at Santa Anita on Nov. 2.
When the park's racing season began Saturday, a handful of animal-rights activists hosted a "funeral'' and vigil'' that included a bagpiper outside the track for the horses that have died over the past year.
The California Horse Racing Board is set to issue a report next month on the horse deaths.
The Los Angeles County District Attorney's Office concluded Dec. 19 that there was no criminal wrongdoing connected to the deaths, but offered a series of recommendations aimed at improving safety at racetracks in California.
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A task force formed by the Los Angeles district attorney found the 49 deaths at the track during a 12-month period ending in June occurred at a rate higher than the national average, but lower than some years in the past decade and lower than Churchill Downs in Kentucky.
Earlier this month, Santa Anita debuted a "cutting-edge'' PET Scan machine to provide imaging of the fetlock or ankle joint - the most common area for injuries to occur in Thoroughbreds -- without horses having to undergo anesthesia, and said it will help to diagnose pre-existing conditions in Thoroughbred racehorses.
RELATED: Santa Anita Park unveils standing PET scan machine for horses
"This state-of-the-art technology reflects a new standard of care within Thoroughbred racing -- a standard that puts the health and safety of horses and riders first,'' said Belinda Stronach, The Stronach Group's chairman and president.
City News Service contributed to this report.