"This is an unprecedented equine medical event," said the association's executive director Peter Rizzo.
"We are actively working with the management of the International Polo Club of Palm Beach, state and local veterinarians and state and county regulatory agencies, including the State Department of Agriculture and Palm Beach County Department of Animal Care and Control, to determine what caused these horses to die," he said.
"While there is much speculation as to the cause of death, we will have to wait until the Department of Agriculture completes its tests and releases its findings to the owners of the horses.
"In the meantime, we all mourn the loss of these horses.
"There are no words to describe the grief and sadness shared by everyone - particularly the devastated owners of those magnificent horses."
The association said there had been much speculation and rumour over the cause of the deaths.
However, the association quoted Dr Paul Wollenman, who it said was on the scene and led a valiant rescue effort with many local veterinarians and polo players to save the stricken horses: "Based on initial, overwhelming clinical evidence this medical event was isolated to the Lechuza barn and horses and the initial evidence shows no infectious element."