Suffolk County police say the crash happened Saturday night in Fort Salonga.
They say 53-year-old Gregory Henig, of Holbrook, was driving southbound when his vehicle hit a car driven by 41-year-old Jennifer Snyder, of Huntington Station, that was headed westbound.
Snyder's car then hit a third vehicle driven by a Brentwood man.
Snyder and her two passengers, her 29-year-old sister, Emily Snyder, and Emily's 4-year-old son, Finn Egan, were taken to a hospital, where Finn later died.
Henig was taken to a hospital with non-life-threatening injuries. The third driver and his two passengers were taken to a hospital and released.
Police are investigating the crash.
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