Washington - A Florida pharmacy that mixed medication for 21 polo horses before they mysteriously died over the weekend said Thursday it incorrectly mixed the substance, the Palm Beach Post reported online.

The horses, which belonged to the Venezuelan team La Lechuza owned by Venezuelan multimillionaire Victor Vargas, died shortly before a quarter-finals match was to begin Sunday at the 105th US Open Polo Championship in Wellington, Florida.

In a related development, the compound biodyl, a vitamin supplement that is banned in the US, emerged as a possible suspect in the horses' collapse. The medicine is mentioned in a letter from a lawyer to the polo team's veterinarian quoted by the Palm Beach Post.

Jennifer Beckett, chief operations officer for Franck's Pharmacy, said in a statement that their own internal investigation showed the incorrect strength of one medication had been used.

She did not name the medication or the flawed ingredient, saying the company was cooperating with the official investigation by state officials and she could not "discuss further details about this matter at this time."

The deaths of the horses has caused a stir in Argentina, a foremost polo-playing country and the place of origin for most of the animals.

Argentina said earlier this week it was considering testing the horses for doping.

"This can undoubtedly lead to the implementation of anti-doping testing in polo. It is expensive and hard to do it, but yes, it is a concrete possibility and it is being evaluated," said Francisco Dorignac, president of the Argentine Polo Association, in published comments.

Experts, breeders and veterinarians in Argentina said the multiple deaths surprised them since the vitamin supplements usually given to polo horses do not have harmful effects - and are usually given after competition, not before.

"The strange thing is that, be they vitamins or whatever, they were given to them before the game, when the normal thing would be to do that afterwards, for recovery," said former polo star Horacio Heguy in Buenos Aires.

Sources close to the team estimated that the horses were worth 1.5 million dollars.

The investigation is being carried out by the Florida State Department of Agriculture, which is conducting post-mortem examinations and toxicology testing on the remains.

Right before the 3 pm match, as the Lechuza Caracas trainers were preparing their horses, two animals collapsed and several others began exhibiting dizziness and disorientation, according to a statement by the North American Polo League.

Despite emergency intervention, the horses all died. (dpa)