Royal Callan Rocks, a 2-year-old colt who had not yet competed in a race, died during training Thursday at Los Alamitos Race Course, state records show.
The horse, trained by Jose A Flores, was training on the dirt track Thursday, but no other information was immediately available.
The California Horse Racing Board reported the death.
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Attempts to reach the track for comment Tuesday were not successful.
Two horses died at Los Alamitos last month: Flokie, a 2-year-old gelding who won the $1,070,000 Ed Burke Million Futurity in September, was killed March 29, and Chickititas Favorite, a 3-year-old gelding, died at the track on March 8.
Radio Tim, a 3-year-old gelding, and Street Machine, a 5-year-old gelding, were fatally injured on Feb. 21, and four horses sustained fatal injuries in January at the race course.
Los Alamitos remains open for racing during the coronavirus pandemic, albeit without fans in attendance. Santa Anita Park in Arcadia closed down its course last week.
CHRB officials said they were "relying on state, county and local health authorities to determine whether horse racing is deemed essential for exemption from shelter-in-place orders issued by those authorities.''
The horse racing board said it would "assist health authorities in enforcing their decisions.''