Investigators said the man, Christopher Lulenski, lived with his girlfriend in a basement apartment on Bonnie Road in Centereach.
The girlfriend's estranged husband showed up at the house and broke a glass door to get inside.
"He had a confrontation with her new boyfriend. That confrontation spilled out into the street. The new boyfriend was beaten to death with a baseball bat," Lt. Det. Jack Fitzpatrick said.
Police said they are looking for 30-year-old Keith Amato in connection with this case.
Amato was last seen in a late-model black Nissan Altima.
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