ARCADIA, Calif. — The free coffee was flowing, the horses were already into their morning workouts, and the sun was not yet up over the San Gabriel Mountains. These mornings at Clocker’s Corner, a breakfast spot for horsemen and other hangers-on at the storied Santa Anita Park, are usually filled with gossip and banter.

On Friday, though, there was only apprehension. A day earlier, the owners of the track announced strict new rules for their sport — no race-day drugs, no use of whips — in response to a spate of horse deaths, 22 since the end of December.

The new rules have not only put trainers and owners at the track on notice, but also convulsed a multibillion-dollar industry from Kentucky to New York that has resisted meaningful oversight for decades. The stakes are high, especially in California, where the animal rights movement is particularly strong, and all it takes is 600,000 signatures on a petition to prompt a ballot initiative on whether horse racing should even exist here.

Even more harrowing is the fact that Los Angeles County district attorney’s office investigators are looking into deaths at the behest of People for the Ethical Treatment of Animals.