The person spoke to The Associated Press on the condition of anonymity Tuesday because the fatality has not been announced publicly.
The person says a filly trained by Hall of Famer Ron McAnally was pulled up during morning training on the dirt track and was taken off by van. The filly was later euthanized because of her injury.
A total of 21 horses have died since the racetrack’s winter meet began on Dec. 26.
Track officials had already announced that Thursday’s racing was canceled and racing won’t resume until Friday, although the track is open daily for training.
Last week, Santa Anita was closed for two days while the dirt surface underwent extensive testing.