A veterinarian who was at the scene said the tests will need to determine the trigger for what he believed was heart failure among the horses.
"Well clearly, it's an intoxication, clearly there's some sort of a poison," Dr. James Belden told NBC.
Belden said it remains to be seen "whether it's something in the environment or something that the horses were exposed to." He said the routine in the horses' stable ahead of the match was absolutely normal.
The polo grounds in Wellington, a wealthy equestrian and golfing community in central Palm Beach County, hosts the U.S. Open every year.
John Wash, the polo club's president of club operations, told reporters Monday that doctors had ruled out any sort of airborne infection. "This was an isolated incident involving that one team," Wash said.
"This was devastating," he said. "It was heartbreaking to see that many horses to get sick all at once."
Veterinarians already at the event quickly tried treating the horses, inserting intravenous lines and trying to cool them down with fans and water. Observers hung blue tarps to shield some of the horses from the crowd's view.
The match in the U.S. Open Polo Championship was postponed and an exhibition game with a substitute team was held in its place.
The carcasses of at least 14 horses were taken to a state agricultural laboratory for necropsies to learn the causes of their deaths.
In addition to those 14, seven more horses died overnight, CBS 12 television in Palm Beach reported. A veterinarian who was at the scene could not immediately be reached early Monday to confirm the higher death toll.
Wash said these types of horses can be worth from $10,000 to $200,000 each.
He also said that because doctors had ruled out an infection, the games will resume Wednesday.
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