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 is 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 Venez-uelan-owned Lechuza polo team began falling to the ground shortly before Sunday's U.S. Open match, shocking a crowd of spectators at the International Polo Club Palm Beach in Wellington.
"On an order from a veterinarian, Franck's Pharmacy prepared medication that was used to treat the 21 horses on the Lechuza Polo team," Beckett said. "As soon as we learned of the tragic incident, we conducted an internal investigation."
She said the report has been given to state authorities.
Lechuza also issued a statement to AP acknowledging that a Florida veterinarian wrote the prescription for the pharmacy to create a compound similar to Biodyl, a French-made supplement that includes vitamins and minerals and is not approved for use in the United States.
"Only horses treated with the compound became sick and died within 3 hours of treatment," Lechuza said in the statement. "Other horses that were not treated remain healthy and normal."
Lechuza also said it was cooperating with authorities that include the State Department of Agriculture and Consumer Services and the Palm Beach County Sheriff's Office.
Biodyl is made in France by Duluth, Ga.-based animal pharmaceutical firm Merial Ltd. and can be given to horses to help with exhaustion.
Necropsies of the 21 horses found internal bleeding, some in the lungs, but offered no definitive clues to the cause of death.