Florida state agriculture officials are inquiring into the deaths, as is the United States Polo Association.
The Venezeulan-based Lechuza team has promised to help official inquiries in any way possible and says it remains baffled as to what caused the deaths of the horses. Local media report that the Palm Beach County Sheriff's Office has also opened an investigation, although authorities have reported no evidence of foul play.
The animals, collectively worth more than $US2 million, began dying around 2pm on Sunday shortly after they arrived at the International Polo Club of Palm Beach to play in a fixture as part of the United States Polo Open.
In all, about a dozen veterinarians converged on the grounds to help the ailing horses, but one by one they died. Fourteen died during the afternoon and evening, and another seven overnight.
Veterinarians at the scene said the horses suffered heart failure, the cause of which has yet to be determined.
The dead horses have been taken to the University of Florida's College of Veterinary Medicine in Gainesville and a state laboratory in Kissimmee.
Scientists are conducting post mortems and tests as part of efforts to identify the cause.