Authorities say 18-year-old Matthew Grimaldi, of Lake Grove, impaled himself on a tree limb in Wading River around 7 p.m. Sunday.
Grimaldi was apparently walking along some bluffs with friends attending a gathering near Hulse Landing Beach when he lost his footing and fell. Neighbors said that one of the people in the group may have been Grimaldi's twin brother.
He landed on the branches of a fallen tree. He was pronounced dead on arrival at a local hospital.
"He's the last person that you would want to hear that this happened to," said his friend Michelle Urrutia. "He was just a very sweet, friendly kid. He was always funny. He had a great sense of humor."
Grimaldi and his friends had been at a house party nearby, according to police.
Friends said Grimaldi was an honors student bound for college.
"It's just something that sends a whole shock wave through the whole community," said Urrutia. "All the way down to the junior high level. Just everyone's devastated."