LOS ANGELES (Reuters) - The Los Angeles County District Attorney’s Office has joined the inquiry into circumstances surrounding the recent deaths of 22 horses at Santa Anita Park, one of America’s premiere thoroughbred racetracks, the agency said on Friday.

Word that local law enforcement was involved in seeking the underlying causes for the spate of deaths came a day after Santa Anita announced an unprecedented ban on the use of drugs and whips in competition there.

Santa Anita suspended racing indefinitely in early March to launch its own investigation into whether track conditions led to a unusually high number of breakdowns.

At that time, a total of 21 horses had been euthanized for injuries suffered in competition or training since Christmas, more than double the number of such fatalities during the entire previous season.

On Thursday, a 22nd death occurred when a 3-year-old filly named Princess Lili B broke both front legs at the end of a half-mile (800-meter) workout. The no-drug, no-whip policy was announced within hours by the Stronach Group, which owns racetrack.

A day earlier, federal legislation was reintroduced to ban race-day medication and increase drug testing nationwide.

The animal rights group, People for the Ethical Treatment of Animals (PETA), urged the prosecutor’s office to open an investigation earlier this month into whether any of the deaths constituted criminal violations of state animal-cruelty laws.

The group was informed on Thursday that the D.A.’s animal cruelty case coordinator had done so, said Kathy Guillermo, a PETA senior vice president.

The D.A.’s office confirmed on Friday that a team from its Bureau of Investigations was assigned to work with the California Horse Racing Board, the sport’s regulatory body, in examining the situation at Santa Anita.

The board’s chief of enforcement, Shawn Loehr, also said in a statement his agency and the D.A.’s office were collaborating in a “joint investigation.”

PETA said it welcomed Santa Anita’s new restrictions as a “watershed moment” for racing, a sport that Guillermo said claims the lives of as many as 2,000 horses annually nationwide.

Guillermo said the medication ban would include anti-inflammatory drugs known as “bute,” which she said have been excessively used to mask horses’ discomfort from pre-existing injuries, keeping them in competition when they should be resting.

It also covers the diuretic Lasix, used flush drugs from a horse’s bloodstream, making them harder to detect in race-day testing, she said.

While Guillermo acknowledged the necessity of examining track conditions to determine hazards that might exist there, she said race industry officials have publicly minimized the role that drugs play in causing fatal injuries to animals.

“It’s a bizarre culture,” she said. “On the one hand they (race horses) are big investments. But on the other hand, they’re not making money if they aren’t racing.”