Police say the boy was found face down in the pool just before 6 p.m.. He was at a family's house for a Fourth of July party.
"It was a Fourth of July celebration, and it just happened in a matter of seconds," neighborhood security guard Raymond Rivera said. "Things like this happen. That's why you have to stay on top of your kids."
Investigators went door to door gathering evidence after the child was rushed to North Shore University Hospital in Manhasset, where he was pronounced dead.
Police are investigating.
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