Twenty-one polo horses which dropped dead during a prestigious international tournament in Florida last weekend had been given incorrect doses of medication, a veterinary pharmacy has admitted.

A memorial service was being held on Thursday for the horses, who belonged to the Venezuelan-based Lechuza Caracas team, at the International Polo Club Palm Beach ahead of the day’s semifinals of the US Open semifinals.

Florida’s Department of Agriculture and Consumer Services is still awaiting results of toxicology tests as it investigates the causes of the horses’ deaths.

But in a statement Thursday, Franck’s Pharmacy in Ocala, Florida, said the strength of medicine it had prepared for the horses on orders from a veterinarian had been incorrect, CNN.com reported.

“We will cooperate fully with the authorities as they continue their investigations,” a spokeswoman said. “We extend our 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.”

Earlier this week, Lechuza captain Juan Martin Nero told an Argentine newspaper earlier that he was certain that vitamins administered to the animals had caused their deaths.

“There were five horses that did not get the vitamin, and those were the only ones that survived,” he said.