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WELLINGTON, Fla. (AP) — Officials were waiting Monday for test results to find out why 21 polo horses from a Venezuelan team fell fatally ill before a match at a Florida tournament. One veterinarian said it looked like some kind of poison was to blame.

The horses from the Lechuza Caracas team were being unloaded from their trailers Sunday afternoon when two collapsed and others acted dizzy and disoriented, according to the International Polo Club Palm Beach. Seven horses died at the scene and the rest while being treated elsewhere or en route to medical care.

A veterinarian who was at the scene said the tests will need to determine the trigger for what he believed was heart failure among the horses.

"Well clearly, it's an intoxication, clearly there's some sort of a poison," Dr. James Belden told NBC.

Belden said it remains to be seen "whether it's something in the environment or something that the horses were exposed to." He said the routine in the horses' stable ahead of the match was absolutely normal.

The polo grounds in Wellington, a wealthy equestrian and golfing community in central Palm Beach County, hosts the U.S. Open every year.

"It could be the water, hay, bedding. We just don't know. When we find out what it is, we will take all the necessary actions," John A. Wash, the polo club's president of club operations, told The Palm Beach Post.

Veterinarians already at the event quickly tried treating the horses, inserting intravenous lines and trying to cool them down with fans and water. Observers hung blue tarps to shield some of the horses from the crowd's view.

The match in the U.S. Open Polo Championship was postponed and an exhibition game with a substitute team was held in its place.

The carcasses of at least 14 horses were taken to a state agricultural laboratory for necropsies to learn the causes of their deaths.

In addition to those 14, seven more horses died overnight, CBS 12 television in Palm Beach reported. A veterinarian who was the scene could not immediately be reached early Monday to confirm the higher death toll.