Officials at legendary Santa Anita racetrack near Los Angeles say they want to conduct all the testing necessary after a 21st horse died at the track since Christmas.
The four-year-old filly suffered a catastrophic fall during morning workouts on Tuesday.
It has been an unusually rainy winter at the track – 406 millimetres have fallen.
The track was closed for two-and-a-half days last week after the 19th horse died. The course was declared fit after testing for deformities and racing resumed. The two more horses died.
Seven of the 21 deaths have occurred during a race on the dirt, five during a race on turf, and nine during training on dirt.