Another racehorse died Sunday at California’s Santa Anita Park, raising the death toll there to 23 since Christmas, officials said.

The fatal injury came two days after racing resumed at the thoroughbred track with new rules aimed at improving horse safety.

The horse that died in Sunday’s race, Arms Runner, suffered a fatal injury after colliding with another horse at a dirt crossing, park officials said in a statement.

The second horse, La Sardane, walked back to her barn with no reported injuries, they said.

“While this incident happened during competition on a track that has been deemed by independent experts to be safe, we are working closely with the California Horse Racing Board to understand if there was anything additional that we could have done to prevent today’s tragedy,” park officials said.

The rash of lethal accidents at Santa Anita, about 20 miles northeast of Los Angeles, began in late December. Racing was suspended there on March 3.

Officials have said they weren’t sure why so many horses were breaking down. Rules regulating the use of whips and medication were introduced earlier this month.

The advocacy group People for the Ethical Treatment of Animals, which has called for a criminal investigation of Santa Anita’s trainers, called the new rules a “watershed moment” and urged other race tracks to adopt similar guidelines.