Officials believe that the "very rapid onset of sickness and death" points to toxins or a drug reaction and also allows them to rule out other possibilities.
"At this time, there is no evidence that these horses were affected with an infectious or contagious disease, as there are no other horses affected at this time," according to Florida Agriculture and Consumer Services spokesman Terence McElroy.
Officials are continuing to conduct tests, and Florida agriculture scientists are performing necropsies on the animals.
Pending test results, there is some speculation as to foul play:
Celeste Kunz, chief examining veterinarian at the New York Racing Association and a 19-year veterinarian, said Monday that she suspected a tainted substance was injected into the horses.
"[It was] something that was administered for it to work in a short amount of time and have an animal succumb that quickly," Kunz said. "My thought is that something was injected, because it would have to affect the central nervous system."
Anabolic steroids are not likely to have caused the deaths, either, Kunz said.
"It takes at least five days for [anabolic steroids] to really work, and the effects aren't real obvious at first," she said. "Most of the time, [anabolic steroids] are used to build up their muscularity."
UPDATE, 4-23: Pharmaceutical error apparently caused the death of the horses