A member of a Venezuelan-owned polo team says a vitamin supplement likely killed 21 horses before a match last weekend in Florida.
Juan Martin Nero, of Lechuza Polo, tells the Argentine newspaper La Nacion the horses were given a commonly used supplement known as Biodyl that contains a combination of vitamins and minerals.
Nero says the drug is often given to horses, but was likely tainted at a lab.
The horses began collapsing Sunday as they were unloaded from trailers before one of the sport's top championships at the International Polo Club Palm Beach.
Nero says five horses that did not get the supplement were OK.
Florida authorities have started a criminal investigation to see if the deaths were intentional or an accident.
Meanwhile, championship polo matches were scheduled to resume in Florida on Thursday, four days after the 21 prized horses mysteriously died.
The International Polo Club Palm Beach plans a memorial ceremony for the dead horses after Thursday's last match at the U.S. Open Polo Championship.
"There are no words to describe the sadness shared by everyone — particularly the devastated owners of those magnificent horses," the United States Polo Association said in a statement issued Wednesday.
Investigators are still stymied as they continue both a criminal and a medical probe. Initial tests revealed internal bleeding but no definitive cause. Authorities hope lab tests will reveal new details.