The alleged assault happened early Friday morning inside a home in Searingtown.
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Detectives say 57-year-old Stacy Sakeran, of Jamaica, was employed as a nanny for the family.
The father of the baby reportedly heard his daughter cry out as if in pain just after 1:30 a.m., prompting the parents to review video from a camera installed in the baby's room.
Authorities say they observed Sakeran forcefully moving the baby around and slapping her in the face. They then viewed video from previous days, and on May 30, they observed the nanny yelling at the infant and striking her several times with an opened hand.
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Police were notified, and Sakeran was placed under arrest just after 4 a.m. Friday.
She is charted with two counts of second-degree assault and two counts of endangering the welfare of a child.
Authorities are working to determine if there are more victims, and anyone with information is urged to contact the Third Squad at 516-573-6354.