Southern California's Santa Anita Park postponed horse racing indefinitely Tuesday so experts could study its dirt surface following the deaths of 21 horses since the Arcadia, Calif., track's winter meet began Dec. 26.

The unprecedented move has shut down two major races scheduled for the weekend, including the San Felipe for 3-year-old Kentucky Derby hopefuls and the Santa Anita Handicap for older horses.

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"In whole, we feel confident in the track and we're just being very proactive," said Tim Ritvo, chief operating officer of the Stronach Group, which owns Santa Anita Park. "We want to do all the testing that needs to be done. When we believe we're in good shape, we'll start to train over it again."

While racing for the week of March 14 remains scheduled, Ritvo wouldn’t speculate on when training and racing would resume at the track, which opened in 1934.

Since the winter meet began, seven of the deaths occurred during races on the dirt oval, five on the turf track, and nine during training on dirt.

The highest-profile horse to be euthanized was Battle of Midway, winner of the 2017 Breeders' Cup Dirt Mile. The 5-year-old bay also finished third in the 2017 Kentucky Derby for Hall of Fame trainer Jerry Hollendorfer. The horse suffered injuries during a workout on Feb. 23.

Lets Light the Way, a 4-year-old filly, was euthanized after shattering the sesamoid, a bone that provides anchor points for the two branches of the suspensory ligament, in her front right leg during training on Tuesday. Hall of Fame trainer Ron McAnally purchased the filly for $15,000.

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McAnally said she “took a bad step or something” and thought the recent rainy weather “has a lot to do with it.”

Last week, Mike Peterson, a nationally renowned surface specialist from the University of Kentucky, performed extensive testing on the track’s composition of dirt and soil and found no irregularities.

The tests used radar to verify that all of the silt, clay and sand, as well as the moisture content, were consistent throughout the track. Its dirt surface was peeled back 5 inches and reapplied.

Two more horses died following those evaluations.

“If you can’t figure it out, you have to quit racing,” Peterson told the Los Angeles Times before the cancellations. “At this point I’m skeptical of what we do. … I’ve told [racing secretary] Steve Lym, keep testing until we know what’s going on. We’re all in. We’re going to figure it out.”

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Santa Anita Park on Tuesday hired its former track superintendent, Dennis Moore, as a consultant to begin evaluating the track Wednesday, the paper reported. The University of Kentucky has also sent another track safety expert to investigate the surface.

In 2017, 20 deaths occurred among a total of 8,463 starts over a span of 122 racing days at Santa Anita, according to the most recent figures compiled by the Jockey Club.

The Associated Press contributed to this report.