WELLINGTON, Fla. (AP) — The Food and Drug Administration has joined a state probe into the mysterious deaths of 21 polo horses from a Venezuelan-owned team.

The horses died over the weekend as they prepared for a match in Florida. State authorities are investigating.

FDA spokeswoman Siobhan DeLancey says her agency has also now joined the probe.

A member of the Lechuza Caracas team tells the Argentine newspaper La Nacion he suspects a tainted supplement given to the horses caused the deaths. The product, Biodyl, is not approved for use in the U.S.

It was not clear whether the horses were given the brand name product or a generic compound created at a Florida pharmacy.

The International Polo Club Palm Beach will resume the U.S. Open games Thursday.