Detectives say the suspect used a cellphone and stood on a covered porch in Elmont to record into the tenant's bathroom.
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At the time, police say the woman's sister, a 6-year-old girl and a 15-year-old girl were in the bathroom.
Police said a neighbor saw the man recording the girls and called the house to alert the homeowner, who then called police.
The suspect fled the scene on foot.
"This is neighbor watching neighbor in this community," Nassau County police Detective Lieutenant Richard LeBrun said. "This individual did the right thing. He called the occupant of the home. He let them know that they had observed a male on the back porch videotaping, and again, police were called right away."
Police have increased patrols in the area to try to track him down.
"Everyone has to keep a nice eye out, a keen eye out, and call the police immediately if they see this individual or anybody that's acting suspicious," LeBrun said.
Neighbors were shocked.
"Watching little kids, how sick can you be?" area resident Lauren Tuomey said.
The suspect is described as a white man in his 20s who was wearing camouflage shorts, a light colored shirt and dark sneakers.
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Anyone with information is asked to call Nassau County Crime Stoppers at 1-800-244-TIPS.
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