The injury occurred at about 8 a.m. According to the Daily Racing Form, the horse was Princess Lili B, a 3-year-old filly. She broke both front legs, the publication reported. It's unclear what caused the fatal injuries.
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The racetrack had just reopened its main one-mile track for limited training Monday.
MORE: Santa Anita Park creates new health and safety rules
Santa Anita Park creates new rules following string of horse deaths
The company that owns the track later announced it is making sweeping changes.
"These changes include a total ban of all medication on race day," said Tim Ritvo of Santa Anita Park.
Officials for the racetrack say they are also banning the use of the controversial drug Lasix and changing the way jockeys use riding crops.
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As for the track: Santa Anita Park spokesman Mike Willman said crews have conducted multiple tests of the soil composition and consistency and are still slated to re-start racing on Monday.
"This racetrack is safe and we intend to move forward, and we intend to move forward in a completely new way," he said.
Racing at the park has been suspended indefinitely after the number of Thoroughbred deaths started to skyrocket.
Kathy Guillermo, senior vice president with People for the Ethical Treatment of Animals, asked the industry to do more.
"Santa Anita's new procedures acknowledge that the deaths ... were never just about the rain and a bad track. They're a step in the right direction -- but they don't go far enough,'' Guillermo said.
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PETA is blaming the horse deaths on owners and trainers recklessly pushing injured animals.
"We know from the thousands of necropsies that have been done on racehorses over the past 20 years that the problem is usually a preexisting injury at the site of the break, and that needs to be looked at. Why are unfit horses being put on the track?" Guillermo added.
PETA is demanding the L.A. County District Attorney's office to start an animal abuse investigation of Santa Anita Park trainers.
Meantime, Santa Anita Park has become the first racetrack in North America to ban race-day medication. PETA later on Thursday thanked the racetrack for its dramatic changes and called on other tracks across the country to follow suit.
City News Service contributed to this report.