WELLINGTON, Fla. — Investigators have opened a criminal probe into whether someone poisoned 21 polo horses that died during preparations for a match in the sport's top championship in Florida, officials said today.

The horses from a Venezuelan-owned team began collapsing Sunday as they were unloaded from trailers at the International Polo Club Palm Beach, with some dying at the scene and others hours later at stables or clinics. State investigators believe the horses died from an adverse drug reaction, toxins in their food or supplements, or a combination of the two.

While state veterinarians run tests to determine what caused the deaths, law enforcement officers are looking into whether criminal negligence, or perhaps something more sinister, could have been involved, said a spokesman for the Department of Agriculture and Consumer Services.

"We want to make sure from a law enforcement standpoint that there was no impropriety ... no purposeful harm or laws violated in Florida," said department spokesman Terence McElroy.

Officials at the club hosting the U.S. Open tournament and several independent veterinarians have said the deaths appeared to be accidental, and investigators haven't identified suspects or foul play.

"We would be looking at any and all possible leads," said McElroy, whose department is leading the investigation with the help of the Palm Beach County Sheriff's Office.

It may take weeks to complete toxicologies to pin down the cause of what veterinarians believe was a swift toxic reaction that killed the horses, which were all from the Lechuza Polo team. Necropsies are also pending.

The deaths of the horses scheduled to play in Sunday's finals could have cost the team's owner, affluent Venezuelan businessman Victor Vargas, more than $2 million, with each horse estimated to be worth $200,000. However, the player-owner didn't stand to lose much in potential winnings, as purses in the sport played largely for pride rarely top a few thousand dollars. There is no purse in the U.S. Open.

The tragedy also shocked Wellington, a town of horse clubs, training facilities, stables, polo grounds and wide-open fenced fields where the animals roam and graze along straight-line, neatly groomed streets.

"The players, the owners of the horses were in tears. Bystanders and volunteers were in tears. I mean, this was a very tragic thing," said Tony Coppola, 62, an announcer for the club, which hosts the tournament each year about 15 miles west of the millionaire enclave of Palm Beach.

Lechuza Polo, a favorite to win the title at the event described as the sport's World Series, released a statement late Monday that the team does not know the cause of the deaths, but is helping with the investigation.

"This is tragic news. We are deeply concerned about the death of our ponies," the statement read. "We have never encountered such a dire situation like this as our horses receive the most professional and dedicated care possible."

John Wash, the polo club's president of club operations, said doctors had ruled out any sort of airborne infection and that the tragedy was isolated to Lechuza's team.

He said games would resume on Wednesday, with the finals taking place Sunday. The Lechuza team has withdrawn.

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Associated Press writer Sarah Larimer contributed to this report.