Officials suspect the horses died from an adverse drug reaction or toxicity
A Florida pharmacy has revealed that the dosage of the medication it prepared for 21 polo ponies that died earlier this week was "incorrect".
Franck's Pharmacy said the horses' medication was prepared "on an order from a veterinarian".
The horses, from a Venezuelan-owned team, collapsed and died at the weekend ahead of the US Open Polo Championship at Wellington in Palm Beach County.
Florida agricultural officials and the sheriff's office are investigating.
Jennifer Beckett, chief of operations at Franck's Pharmacy, said an internal investigation had revealed that "the strength of an ingredient in the medication was incorrect".
The statement did not specify which medicine or ingredient was thought to have killed the horses.
The pharmacy said it would "co-operate fully with the authorities as they continue their investigations".
The horses, valued at $100,000 (£69,000) each, belonged to the Lechuza Caracas polo team, which competes in polo tournaments around the world.
They first developed symptoms as they were being unloaded from horseboxes at the polo club in Wellington.
Fourteen horses had died by Sunday evening, with seven more dying overnight.
Ms Beckett said the pharmacy extended their "most sincere condolences to the horses' owners, the Lechuza Polo team and the members of the United States Polo Association".
"We share their grief and sadness," she said.
Lechuza Polo have released a statement saying that a vitamin supplement given to the horses could have been to blame.
"Only the horses treated with the compound became sick and died within three hours of treatment. Other horses that were not treated remain healthy and normal," they said.
Officials investigating the incident suspected that an adverse drug reaction or toxicity was behind the horses' death.