ARCADIA, Calif. — There is a mystery unfolding along the rail here at one of the nation’s most storied horse tracks, where since Christmas nearly two dozen peak-of-form thoroughbreds have died during races or training at Santa Anita Park.

No one knows why.

The unusual spate of “breakdowns,” as broken legs are known in horse racing, has focused harsh attention on the sport, particularly the way it treats its prize equine athletes, just before the Triple Crown season begins next month at the Kentucky Derby.

“To a novice, it must be very shocking,” said Robert Leon, 72, who mapped out his picks one warm recent morning at the park, as he has for several decades.

Leon watched as Arms Runner, a 5-year-old gelding, broke down March 31 on the track’s unique downhill course, crashing as he descended the banked turf turn onto the dirt. The horse was subsequently euthanized — the most recent of 23 horses to die here in recent months — and the downhill course was closed indefinitely.

“There’s nothing we can do about it,” Leon said. “This is up to the people who know horses.”

The deaths, now the subject of postmortem exams, have amplified calls from animal rights advocates to shut down California’s more than $3 billion annual horse racing industry until the mystery of Santa Anita is solved and new rules are in place to reduce fatalities.

The track halted racing for nearly all of March to look into the deaths before reopening March 29. Next week, the California Horse Racing Board, which oversees the industry, is scheduled to meet here to discuss whether the remainder of the track’s season should be transferred to another venue. Sen. ­Dianne Feinstein ­(D-Calif.) wrote the board April 2 calling for racing at Santa Anita to be ­suspended immediately.

“The death of a single horse is a tragedy,” she wrote, “but as a lifelong lover of horses, I’m appalled that almost two dozen horses have died in just four months.”

Track owners worry that without reform horse racing may go the way of Ringling Bros. and Barnum & Bailey Circus, which after more than 140 years as “The Greatest Show on Earth” closed in 2017 amid concerns about the way it treated its animals.

“As the recent tragedies at Santa Anita have illustrated, thoroughbred horse racing in the United States is at a crossroads,” the Stronach Group, which owns Santa Anita and Pimlico in Baltimore, among other tracks, said in a statement. “The fact that horses running in America are five times more likely to suffer a catastrophic injury than horses running in international venues is unacceptable and must ­immediately change.”

History and horses

Stretching out at the foot of the San Gabriel Mountains east of Los Angeles, Santa Anita Park is the most famous of California’s horse racing venues, an annual proving ground for the Triple Crown where the pressure to perform is always high.

Some of the greatest thoroughbreds in history have run here since its founding in 1934. But its lush infield and clubhouse, a fantasy of balconies, porches and filigree ironwork, also served as the nation’s largest “assembly center,” a way station for Japanese Americans on their way to internment camps during World War II.

The economy and culture of this city pivot around the park.

Along Huntington Drive, its storefronts advertising derby hats for sale or rent, the sidewalk is inlaid with stars such as those along Hollywood Boulevard. They celebrate the trainers, jockeys and horses — Alysheba, ­Affirmed, Citation — who have competed in the Santa Anita ­Derby.

While glamorous, Santa Anita is also the state’s most dangerous track. From the start of July 2017 through June 2018, 44 horses died here, according to the state horse racing board.

Mike Marten, a spokesman for the board, said that even if deaths at the park “even out statistically this year, this number of fatalities so close together is very unusual.” The number of horse deaths at Santa Anita since Dec. 26 is more than double those recorded at the park over the same period in each of the past two years.

“People are concerned, and I don’t blame them,” Marten said.

The trend line for horse racing fatalities in California has sloped down from 2004, when 320 thoroughbreds died running on state tracks.

Hundreds of racehorses die each year. Their deaths may be preventable.

Meetings involving board members, trainers, owners, jockeys and others in the summer of 2016 tightened rules around day-of-race drug administration, whipping and other measures designed to discourage the use of performance-enhancing techniques, including pain management. The following year, Marten said, the number of horse deaths dropped to 138 statewide.

“That’s what makes this all the more frustrating,” he said. “We did some things we thought would help, and I know the ­commissioners are feeling very frustrated and dismayed by this.”

Thoroughbreds are freakish athletes with huge, muscular bodies atop thin, sinewy legs and a massive heart that pumps 10 gallons of blood through it all. The average adult human has between a gallon and a gallon and a half of blood.

Susan Stover, a professor of anatomy and physiology at the University of California Davis School of Veterinary Medicine, said despite the heavy-on-top appearance “thoroughbred racehorses are not inherently weak or highly susceptible to injuries under normal circumstances.”

“Just like any other elite athlete that performs an activity at an intense level repetitively, racehorses can incur injuries,” she wrote in an email response to questions. “Racehorses become susceptible to injuries when the intensity of training exceeds the ability of the skeleton to recover from training-induced damage between bouts of exercise.”

Weather concerns and more

A number of factors might be contributing to the catastrophic horse injuries at Santa Anita.

Decision to shut down track: ‘Something is not right’

The main difference between this season and last is the weather, the state’s wettest in a decade. Rainfall has far exceeded averages, giving the San Gabriel Mountains deep green slopes when a dusty tan is the more normal shade.

In 2008, the state horse racing board required all tracks to replace the traditional dirt with a synthetic surface. The change was adopted in the name of ­safety, but within a few years all but one California track had moved back to dirt.

Santa Anita was among the first to do so, arguing that track drainage was poor and the surface dangerous as a result. Management claimed horse trainers were threatening not to race there until changes were made. In Marten’s words, “It just didn’t work.”

“Most of the horsemen were happy to get back to dirt,” said Terry Meyocks, president of the Jockeys’ Guild, which represents thoroughbred and quarter-horse riders. “You hate to say what is happening at Santa Anita now is a perfect storm, but some of what is contributing is weather that the track is just not accustomed to.”

As measured by fatality rates, turf usually emerges each year as the safest racing surface. It is followed by synthetic, found in California now only at Golden Gate Fields (also owned by the Stronach Group) in the East Bay. But dirt is the fastest surface, often favored by owners and trainers.

Bob Baffert, a two-time Triple Crown-winning trainer, said the makeup of the track at Santa Anita may have been scrambled by the heavy rain, making it more of a hard clay with too much sand washing out.

“I really don’t have concerns about the track,” said Baffert, whose horse Roadster won the million-dollar Santa Anita Derby on April 6, with his horse Game Winner finishing second. “Believe me, if I had an issue, I wouldn’t be running here.”

Analysis: Best bets for the Santa Anita Derby

Baffert said the rain and rash of deaths were “like a storm that wouldn’t pass.” In recent weeks, he said crowds and the media were turning out at the park with the morbid curiosity of “watching a car chase in Los Angeles.”

“What I worry most of all about are the families that have been here for generations and who really rely on this racing,” Baffert said. “When an industry like this shuts down, you’re talking about 30,000 to 40,000 people affected. You don’t want to see those jobs disappear.”

But Baffert said a “remixed” track that aimed to better balance the dirt, sand and clay appears to have improved safety, given the lack of deaths on the dirt-only main track since March 14, when a filly named Princess Lili B broke her front legs during a workout.

“I feel like they have everything under control right now, but I don’t want to jinx myself,” Baffert said. “It’s going to happen again, but not to the extent it has during this recent period.”

'It's sad, just awful'

Veterinarians and animal rights groups said there is more to the deaths than a prolific rainy season and track maintenance, reasons that are more entwined with long-standing racing practices and culture.

“Compaction of the surface can make the surface harder, increasing loads to the horses’ limbs,” wrote Stover, the UC Davis professor. “However, it is unlikely that the surface was solely responsible for the rash of catastrophic injuries. Inherently healthy horses may not have had injuries on a racetrack managed to handle the rain.”

Race-day drugs and the jockey’s whip are horse racing staples receiving new scrutiny. Both are used to drive thoroughbreds — sometimes beyond their physical capabilities — to win.

After the 22nd horse death, the Stronach Group announced a unilateral ban on certain drugs and cut in half the allowable day-of-race dosage of Lasix, a diuretic and anti-bleeding medication that also can be used to mask the presence of performance-enhancing drugs. The company also announced limits on whipping, a regulation on hold until the state board adopts similar rules for tracks statewide.

“This is a great opportunity to clean up racing in California,” said Kathy Guillermo, senior vice president at People for the Ethical Treatment of Animals . “Let’s get rid of everything that causes horses to suffer.”

Guillermo said horse necropsies, as the postmortem exams are called, often find that leg breaks occur at the site of a previous injury or current fracture. Drugs can mask these injuries, intentionally or not, in addition to others that may stimulate performance.

PETA is calling for a ban on all medication in racing and a return to synthetic tracks, which Guillermo said the industry largely opposes because they are slower. Until those rules are in place, she said, horse racing in California should shut down.

“We think there may be multiple reasons for the deaths, but what cannot be denied is that injuries and medications certainly play a role,” she said. “Santa Anita had the obligation to stop and look at everything during the rains. Now that the rains have stopped and we await the results of the necropsy reports, they have the obligation to wait.”

As post time approached on a recent Friday morning, the televisions around Santa Anita were tuned to racing at Aqueduct Racetrack and Gulfstream Park, both on the other side of the country. A head wind along the backstretch stiffened the large American flag flying over the infield, bending the beanstalk palms and kicking up a thin veil of dust over the track.

Andrew Murphy, a 43-year-old insurance salesman, and his father, Frank, make the trip from Florida to Santa Anita for derby weekend every year. The two placed bets on the remote races and charted what they hoped would be a winning day at Santa Anita when, on the TV screens above the bar, a horse broke down in the seventh race at Aqueduct.

“I’ve been playing this game since I was 5 years old, and it certainly isn’t for everybody,” Murphy said. “It’s sad, just awful. But it’s part of the game.”