After hearing screaming in the backyard, neighbors called police who responded to the home on Clearview Road shortly after 4 p.m. Monday.
Police said an 8-year-old girl, her 32-year-old mother, and her 62-year-old grandfather drowned. Authorities have not released their names.
Neighbors said the family had recently moved into the home and were friendly.
Authorities called an electrical contractor to the home and sources said they believe there may have been an electrical issue in or near the pool that led to the deaths. Police have not confirmed that theory.
"This is a devastating day for our entire community. It is too early to determine exactly what happened," Police Chief Frank Losacco said in a statement. "We are working with the Middlesex County Prosecutors office and will release information as it develops."
Other family members were said to be inside the home at the time of the incident.
The investigation was ongoing.
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The Associated Press contributed to this report