Rescuers were still working to recover the body, Demings told reporters.
"We're 15 hours in," Demings said. "We know that that happened and it certainly is not survivable at this point for him to have been submerged for this period of time."
"There's no question... (the family) will lose a two-year-old child," he said. The family of five had come to Florida from Nebraska for a vacation.
The mauling was yet more terrible news for a vacation town still reeling from the weekend shooting massacre of 49 people at a gay nightclub in downtown Orlando -- the worst mass shooting in American history. Another 53 were wounded.
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"What I can tell you is that Disney has operated here for 45 years and never had this type of thing happen before," Demings said.
"Remember, it is Florida. And alligators are indigenous to this region of the country. But I want to reiterate that Disney has a wildlife management system that is in place and they have worked diligently to ensure that their guests are not unduly exposed to the wildlife here in this area," he said.
Authorities said earlier four alligators were removed from the lake and euthanized, and Wiley said a fifth had been found.