A fourth horse has died at Del Mar following an injury during training on Monday, racetrack officials said.
The three-year-old filly named Bri Bri was euthanized after suffering a serious injury to her pelvis, Del Mar Thoroughbred Club (DMTC) officials said.
The circumstances leading to the injury were not disclosed.
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DMTC said track veterinarians were on-site at the time of the accident and responded immediately. Officials would be meeting with the horse's trainer to discuss the incident.
The death marks the fourth horse fatality at the tracks since the racing season started on July 17.
On July 30, a 3-year-old filly Bowl of Soul, trained by Bob Baffert and owned by Spendthrift Farm, was put down after a training injury to her back right fetlock, similar to a human ankle.
On the second day of the season, two horses died following a head-on collision during training.
One of the horses made a sudden U-turn and collided with another galloping in its direction.
Both horses suffered cervical fractures and died on the racetrack. They were not euthanized, according to DMTC officials.
Carson Valley, one of the two horses involved in that collision, was also trained by Baffert.