WEST PALM BEACH, Fla. (AP) – Twenty-one prized polo horses that mysteriously died before one of the sport’s top championships were given a supplement that likely caused their deaths, the leader of the Venezuelan-owned team told an Argentine newspaper.

Juan Martin Nero, of Lechuza Polo, told the newspaper La Nacion for Wednesday’s editions that the horses were given a supplement known as Biodyl, which contains a combination of vitamin B12, a form of selenium called sodium selenite and other minerals.

It is made in France by Duluth, Ga.-based animal pharmaceutical firm Merial Ltd. and can be given to horses to help with exhaustion. It is not approved for use in the United States, but is widely used elsewhere.

It remained unclear whether the Lechuza team used the brand name drug or a copy of it made by a compounding pharmacy in the U.S., which the Food and Drug Administration said could be illegal. 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. They cannot legally recreate a drug that is not approved in the U.S.

Florida authorities have started an investigation to determine if the deaths were intentional or accidental, but so far nothing criminal has surfaced. Toxicology tests are pending, and officials said they would add Biodyl to the list after hearing the reports from Nero.

“For us, the suspicions are that there was something wrong in the laboratory. They are common vitamins, that are not given to improve performance, but so they recover from exhaustion,” Nero told the newspaper in Spanish. “We don’t have doubts of the origin of the problem. There were five horses that were not given the vitamin and they are the only ones that are fine.”

The horses began collapsing Sunday as they were unloaded from trailers at the International Polo Club Palm Beach. Some died at the scene, others hours later. The team was seen as top contenders for the U.S. Open tournament.

“What was given to them is the same as always, vitamins,” Nero said. “Once a week. It was just the time to give them the dose the day of the match.”

The Associated Press has been unable to reach Nero or the team’s owner, prominent Venezuelan banker Victor Vargas, who has not spoken publicly since the deaths.

Merial spokesman Steve Dickinson said the company is confident its product is safe.

“We’ve had reports of pharmacies compounding a copycat Biodyl in the past, which is of course illegal,” Dickinson said.

He said the company has recorded adverse reactions in just about one in more than two million doses over “many years.” He couldn’t immediately say whether any led to death.

Dickinson said the product has been around for more than 50 years.

Necropsies of the 21 horses found internal bleeding but offered no definitive clues to the cause of death, said Sarah Carey, spokeswoman for the University of Florida’s College of Veterinary Medicine, which did some of the tests.

A veterinarian who works with polo horses said giving the animals vitamin supplements is common practice.

“In the world of polo, it is customary to medicate horses to assist them in recovery from their extremely strenuous work,” Rob Boswell said. “It is not customary to give them substances prior to playing to enhance their abilities. That’s dangerous.”

Boswell said it’s also common practice to use compounding pharmacies in the animal industry, but only reputable ones.

“They don’t have to follow the same quality control practices that an FDA approved drug would have to,” he said.

High doses of selenium, for instance, could be fatal.

“The big question,” he said, “is are we talking about the manufactured product or is it a compounded product? That’s huge.”

Experts said if the supplement was made in the U.S. at a compounding pharmacy, there could have been an error in the lab.

FDA spokeswoman Siobhan DeLancey said compounding pharmacies are legal, but they are not allowed by law to recreate existing drugs or supplements under patent or to recreate any drugs not approved for use in the U.S., such as Biodyl.

The FDA refused to approve Biodyl for use in the U.S. in October 2008, saying it appeared “to be a new animal drug which is unsafe.”

A pharmacy could face criminal charges if it made the supplement, DeLancey said.

The law does not, however, prohibit veterinarians from purchasing and administering each supplement separately.

AP-ES-04-22-09 1847EDT