David Forbes, 38, of Columb, was last seen at Birchfield Court in Columbine at about 11.30pm on Tuesday 4 July.
According to Suffolk County Police, Forbes has autism and diabetes and may require medical attention.
He is 5-foot-10, 170 pounds, and has brown eyes. He was last seen wearing a black backpack.
Anyone with information is asked to call the Suffolk County Police Department at 631-854-8614 or 911.
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