"A comprehensive evaluation will be completed to understand what might have caused this uncommon injury," the owners of the California track said.
Another horse died at Southern California's Santa Anita Park on Friday morning, the 24th death at the track since late December. The animal suffered what was described by park officials as an "uncommon" shoulder injury.
Commander Coil, a 3-year-old gelding, was euthanized after suffering the injury while galloping during training, said the Stronach Group, the company that owns Santa Anita Park.
"Equine shoulder injuries are rare, especially for a horse that is galloping as opposed to breezing or racing," the group said in a statement. "A comprehensive evaluation will be completed to understand what might have caused this uncommon injury."
The injury occurred just before 10 a.m. on the one-mile main track, the group said.
A series of injuries at Santa Anita Park that resulted in horses being euthanized began Dec. 26.
Before Friday, the most recent death happened March 31 after Arms Runner, a 5-year-old gelding, collided with another horse at a dirt crossing during the San Simeon Stakes and suffered fatal injuries, the park said in a statement at the time.
Santa Anita Park on March 5 announced it was suspending racing and training after 21 horses over the course of several weeks suffered injuries that required them to be euthanized, as track officials tried to figure out what was going on.
A 22nd horse was put down following a training accident March 14, prompting the Stronach Group to ban the use of drugs on race days and to sharply limit jockeys' use of whips.
Racing resumed in late March, and two days later Arms Runner suffered the fatal injury. Santa Anita Park called the death of Arms Runner "a gut-wrenching blow" and said it "speaks to the larger issue of doing all that we can to better understand and prevent such catastrophic injuries, not only at Santa Anita but throughout California and across the country."
The Stronach Group said Friday's incident was the first horse death during training since the reforms were put in place on March 15, and there had 80,000 gallops during training with no fatalities.
Santa Anita Park is in Arcadia, about 20 miles northeast of Los Angeles. It is considered one of the most prestigious venues in horse racing, and the Santa Anita Derby is part of the Road to the Kentucky Derby. The 2003 movie "Seabiscuit" was filmed there.