Nassau County police say the boy's grandmother found him about noon Saturday in a pool behind the house in Laurel Hollow.
Detective Lt. Kevin Smith said that the boy had walked away while his older siblings ate lunch.
Smith says investigators believe the 3-year-old walked through the garage and got into the in-ground pool through an unlocked metal gate.
When the grandmother noticed the boy had gone missing, she went looking for him and found him in the water. Authorities were called about 12:05 p.m.
The boy was taken to a nearby hospital where he was pronounced dead about an hour later.
Police are calling it an accidental death.
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