Naugatuck police responded to the Hop Brook Lake Dam after receiving a report that someone might have fallen into the river just after 11:30 a.m. Wednesday.
The Naugatuck Fire Department, Waterbury Police, and Middlebury Police assisted them.
A state police dive team was also brought in for additional resources in the search.
"Earlier this afternoon the Naugatuck police department received a phone call from a concerned citizen who had seen a person in the vicinity of the dam here at Hop Brook. Based upon the info they gave us it was concerning enough that working with the fire department and the Region 5 dive team we launched a search to see if in fact there was a person who had gone into the water at this location," Dep. Chief Colin McAllister said.
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Police were also searching areas down the river as far south as Porter Avenue, according to McAllister.
The search was suspended around 4:30 p.m. after crews didn't find anything.