An official at a Florida pharmacy said Thursday the business incorrectly prepared a supplement given to 21 polo horses that died over the weekend while preparing to play in a championship match.

Jennifer Beckett of Franck’s Pharmacy in Ocala, Fla., told The Associated Press in a statement that the business conducted an internal investigation that found “the strength of an ingredient in the medication was incorrect.” The statement did not say what the ingredient was.

Beckett, who’s the pharmacy’s chief operating officer, said the pharmacy is cooperating with an investigation by state authorities and the Food and Drug Administration.

The horses from the Venezuelan-owned Lechuza polo team began crumpling to the ground shortly before Sunday’s U.S. Open match was supposed to begin, shocking a crowd of well-heeled spectators at the International Polo Club Palm Beach in Wellington