Florida ? The Food and Drug Administration joined state and local investigators Wednesday after a polo captain revealed that 21 horses that mysteriously died in Wellington may have been given a substance banned in the United States.

Top Lechuza Caracas polo player Juan Martin Nero told an Argentine newspaper the horses that died Sunday and Monday were given a weekly vitamin compound called Biodyl. The mixture, a combination of vitamins and minerals, is not approved in the United States and is illegal to possess or use, according to the FDA.

Though Biodyl by itself is unlikely to have killed the prized horses, veterinary experts say, it may have been part of a deadly, home-brew concoction whose legality could also be in question.

The horses went into distress Sunday afternoon at the International Polo Club Palm Beach fields in Wellington. Fourteen died Sunday, the others Monday.

Nero told the newspaper La Nacion that the horses were given Biodyl on Sunday to help them recover from the wear and tear of the polo match.

“We have no doubt about the origin of the problem,” Nero told the newspaper. “There were five horses that were not given the vitamin and they’re the only ones that are fine.”