The victims have been identified as Robert Mark, 66, and Susan Quagliano, 57, who both lived at the same address in Oakdale.
The six-seater aircraft had left MacArthur Airport in Ronkonkoma at 9:00 a.m. en route to New Bedford, Massachusetts when it encountered mechanical problems.
The Beechcraft Bonanza A36 came down on the North Fork in Mattituck at the Harbes Family Farms Field around 9:15 a.m., according to the FAA.
Firefighters, police officers and rescue personnel arrived at the scene within minutes and put out the ensuing fire.
The plane was severely damaged, said Shelley LaRose-Arken, the airport's commissioner.
The official cause of the crash remains under investigation.
Riverhead Police said Sound Avenue will be closed between Herricks Lane and Aldrich Lane during the investigation.
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