An unraced horse named River Derby was injured in training Thursday at Santa Anita Park and became the 27th horse to be euthanized since the start of the racing season in late December at the famed California racetrack.
A spokesman for the California Horse Racing Board, Mike Marten, said veterinarians at an equine hospital in Chino determined surgery was not a viable option.
The horse was galloping during a morning training session at the track in Arcadia when it was injured, the board said.
"The board regrets the loss of any horse. We are committed to working with the entire California horse racing industry in the protection of horses and riders," officials said.
CNN reached out to the owners, the Stronach Group, but didn't receive an immediate reply. Santa Anita Park officials also didn't respond to CNN's request for comment.
Four horses have died since May 17.
On its website, the track owners say trainers must apply 48 hours in advance for permission to give a horse a high-speed training exercise. The track's veterinarians will assist in identifying horses that are at risk for injury, it says.
Horse are only allowed "therapeutic medication with a qualified veterinary diagnosis from a state-licensed veterinarian," it says.