WEST PALM BEACH, Florida — A Florida pharmacy said Thursday that it incorrectly prepared a supplement given to 21 polo horses that died over the weekend while preparing to play in a championship match.

Unable to legally bring a supplement into the U.S. to make their horses more resilient, a Venezuelan polo team had the pharmacy mix up the concoction.

What happened next, though, was disastrous. The chemicals were mixed wrong, and the 21 horses given the brew died in rapid succession, some collapsing just before taking the field in a championship polo match. The others fell soon after, one by one, shocking a well-heeled crowd gathered to watch the U.S. Open at the International Polo Club Palm Beach in Wellington.

The Lechuza polo team had hoped to get a compound similar to a name-brand supplement used safely around the world to help horses with exhaustion but hasn't been approved by the Food and Drug Administration. Veterinarians commonly turn to compounding pharmacies for medications that can't be found on shelves, but the dispensaries can only recreate unapproved drugs in limited circumstances.

A Florida pharmacy that mixed the medication said Thursday an internal review found "the strength of an ingredient in the medication was incorrect." Jennifer Beckett, chief operating officer for Franck's Pharmacy in Ocala, Florida, would not say whether the incorrect amount was specified in the order that came from a Florida veterinarian.

Lechuza said the order was for a compound similar to Biodyl, a supplement that includes vitamins and minerals. The team has been using the supplement for many years without problems, but typically uses the manufactured version instead of going to compounding pharmacies.

"Only horses treated with the compound became sick and died within 3 hours of treatment," Lechuza said in a statement. "Other horses that were not treated remain healthy and normal."

While Biodyl isn't approved in the U.S., the supplement made in France by Duluth, Georgia-based animal pharmaceutical firm Merial Ltd. is widely used abroad. The president of the Agentine Equine Veterinarian Association, Fernando Ruiz, said the supplement is commonly used on horses that compete there, and he's not aware of any deaths.

It wasn't clear how closely Franck's mixture came to the name-brand drug, though. Lechuza said what they ordered was supposed to contain vitamin B, potassium, magnesium and selenium, a mineral that can be toxic in high doses.

Compound pharmacies can, among other things, add flavor, make substances into a powder or liquid or remove a certain compound that may have an adverse reaction in different animal species.

FDA spokeswoman Siobhan DeLancey said the agency's interest is now "heightened" with news the deaths could have been caused by a medical mistake at a pharmacy — one that not only produces drugs for animals, but also people.

Florida's State Department of Agriculture and Consumer Services and the Palm Beach County Sheriff's Office are also investigating the deaths, and the pharmacy and polo team said they're cooperating.

The state agriculture department wouldn't comment on the latest news, but said testing for chemicals in the horses' blood and tissue continued. They hoped to have some results by Friday. Necropsies of the 21 horses found internal bleeding, some in the lungs, but offered no definitive clues to the cause of death.

On its Web site, the FDA says it generally defers to state authorities to regulate compounding of drugs by veterinarians and pharmacists but would "seriously consider enforcement action" if one of the pharmacies breaks federal law. It isn't yet clear Franck's broke the law. The pharmacy has had no complaints lodged against it, according to the Florida Department of Health.

A veterinarian not involved in the case said the laws pertaining to compounding are unclear, and there is little oversight.

"It's confusing to all of us," said Miami veterinarian Zachary Franklin. "We're not lawyers, we're veterinarians.

"Almost no one follows the exact letter of the law," he added.

Franklin said veterinarians often turn to compounding pharmacies to recreate drugs such as antibiotics, but it is much less common to compound vitamin and mineral supplements, because the ingredients are usually readily available.

"I don't know what it is about this Biodyl that they like so much," Franklin said. "There probably is no good scientific reason to do that."

While polo's U.S. governing body doesn't test horses for drugs, officials in horse racing wouldn't bother checking for the ingredients of Biodyl, said the head of a group that helps develop policies for regulating the racing industry.

"There's nothing in it that would be worth testing for in terms of performance," said Scot Waterman, the executive director of the Racing Medication and Testing Consortium. "It's B vitamins and a mineral."

He said there's some concern in his industry about compounding pharmacies, which can be difficult to monitor.

"There are FDA rules on what can and cannot be compounded but there is little oversight," Waterman said. "They play a very important role for the equine practitioner but there is also potentially a dark side to the compounders."

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Associated Press Writers Will Graves in Louisville, Kentucky, Kelli Kennedy in Miami and Vicente L. Panetta in Buenos Aires contributed to this report.