Investigators are trying to determine what killed 21 polo horses which died suddenly just before a tournament in Florida on Sunday.
The horses belonged to a Venezuelan team which competes in England and were worth about $US2 million.
They collapsed before a match at the Palm Beach Polo Club.
The match was delayed and the animals began to die one by one throughout the afternoon and evening.
Team veterinarian James Belden says the cause is almost certainly something they consumed. He says it could be the water, hay, or bedding.
Toxicology tests were to be held Monday.