Officials at Santa Anita Park racetrack were forced to euthanize the 37th horse since December after Mongolian Groom broke his hind leg in the final race of the two-day Breeders’ Cup competition.

“Mongolian Groom sustained an injury in the Breeders’ Cup Classic today and was immediately attended to by an expert team of veterinarians, led by board certified veterinary surgeon Dr. Ryan Carpenter. During their evaluation at the equine hospital at Santa Anita, they observed a serious fracture to his left hind limb,” said officials with the Breeders’ Cup in a prepared statement released after the race.

According to the statement, veterinarians “recommended humane euthanasia of Mongolian Groom.”

The latest incident leaves Santa Anita in a troubling position.

Horse racing resumed on Sunday with a District Attorney’s investigation looming and amid protests and calls by animal rights advocates to shut the track down.

“We want a complete ban on horse racing in the state of California and hopefully the entire United States,” protester Amanda Lundberg told ABC News. “As long as horse racing is still here, horses are going to be killed.”

Just four days before the event, Sen. Dianne Feinstein issued a dire warning to officials at the Santa Anita track.

“I believe this weekend presents a critical test for the future of horse racing in California and in the United States,” Feinstein wrote in a letter to Rick Baedeker, executive director of the California Horse Racing Board. “If horse racing cannot be conducted in a safe and humane manner that protects the life and safety of horses and jockeys, it may be time to reexamine the future of this sport in our state and in our country.”

Members of People for the Ethical Treatment of Animals (PETA) said the group “will not tolerate another mangled horse.”

“Get the drugs out, install CT scan equipment at every track, replace dirt tracks with synthetic, and ban whipping,” said a statement issued by the group. “This is now non-negotiable: The racing industry must make a choice between doing right by the horses or shutting down forever.”

Santa Anita closed briefly after the 21st horse died in March. After the track reopened, officials put new safety protocols in place to protect the animals, including banning race-day medication, increasing transparency of veterinary records, further regulating approval for workouts, and increasing reviews of the track’s surface.

Santa Anita is currently under investigation by the Los Angeles County District Attorney’s Office, a probe launched in March that remains ongoing.

“American horse racing is broken and its equine athletes are paying the price with their lives,” the United States Humane Society said in a statement. “The decision to hold the Breeders’ Cup at Santa Anita before the results of investigations into the spate of recent deaths was ill advised. We believe that a suspension of racing at this track is warranted until the investigations of those deaths, and now the death of Mongolian Groom, are completed and released.”

Under state law, the California Horse Racing Board can only suspend horseracing at a track after a 10-day process and a public meeting, even when there are significant safety issues.

Senate Bill 469, authored by Sen. Bill Dodd, would make it easier to suspend the sport. In May, Feinstein and Gov. Gavin Newsom supported that bill.

In a September interview with The New York Times, Newsom excoriated the so-called sport of kings.

“I’ll tell you, talk about a sport whose time is up unless they reform,’’ Newsom told the Times. “That’s horse racing. Incredible abuses to these precious animals and the willingness to just to spit these animals out and literally take their lives is a disgrace.”

Experts so far have been unable to come to a consensus on what is causing the equine deaths. Some experts initially claimed the injuries were caused by an extra hard track created by track officials in response to the above average rainfall the area experienced in the first three months of the year. However lethal injuries continued after the rainy season ended. Others pointed to drugs and abuse by trainers and owners.

Horses have light, fragile bones that tend to shatter instead of fracture like human bones. A horse’s leg contains 80 of the 205 bones in the animal’s body, according to the National Livestock Journal. Horses have no muscles below their knees, an intricate system of tendons and ligaments powered by tremendous muscles high in the body, allowing the animals to sprint at high speeds.

Because equine circulation is dependent on the horse’s hooves, keeping a horse still for a long time puts the animal’s life at risk and forces owners to have hundreds of injured horses euthanized every year.