18-year-old Kyle Underhill's parents called police Thursday to let authorities know something was wrong.
On Saturday morning, the recent high-school graduate's body was found in a marsh in Islip.
Suffolk County Police say Kyle Underhill was last seen in the early evening on Wednesday.
By Friday, detectives began a full-scale search.
"In the course of the investigation it was determined that there may be suspicious circumstances that were involved," said Det. Lt. Gerard Pelkofsky from the Suffolk Police Homicide Squad.
Kyle's friends took to facebook.
'I really hope ur ok' one wrote.
'Can u please call me when u get a chance,' 'where r you bro' were some of the other postings.
"We're all just confused about it," said Islip High School student Cassie Blumstein.
"It was upsetting. He's so young," said Islip High School student Toni Macken.
The mysterious disappearance of the 2011 graduate became the talk of Islip High School.
Many knew Underhill from his senior yearbook inscription.
"To all my friends, I'm so glad for the times we had together. If we don't ever see each other again, I'll miss you dearly. To everyone else, I hate you all. The best part of graduating is knowing I'll never have to see any of you again. I don't think I've ever known so many people I hated so much all at once. See you in Abaddon."
Abaddon is also sometimes called the depths of hell.
Saturday morning, police found Underhill dead in a marshy area off Brook Street in Islip.
"The body was found in a wooded area, but I'm not going disclose exactly how or what we see," Det. Lt Gerard Pelkofsky said.
Police believe Underhill may have been murdered.
How he was killed and why remain the big questions.
Detectives are asking anyone with information about this incident to call the Homicide Squad or call Crime Stoppers anonymously at 1-800-220-TIPS.