A young Long Island man under investigation by cops for a fracas at a local bar and a nearby break-in was shot and killed by officers when he came at them with a knife, authorities said.

Kurt Doerbecker, 23, of the tiny beachfront community of Point Lookout was shot and killed early yesterday after a standoff at his parent’s waterfront home in Point Lookout, after the youth hid inside and refused to talk to cops.

Nassau Detetctive Lt. Kevin Smith said the youth had twice been ejected from a nearby bar 90 minutes earlier because he had too much to drink and was disruptive. The third time he returned and made threats, someone at the bar called police.

Later, a block away, Doerbecker burst into a home in a mask, apparently carrying a sharp object, and confronted the homeowner – who said she yelled at him to get out, which he did, said Smith.

Doerbecker refused to speak to police at his house and his parents refused cops entry. Just after midnight yesterday morning, the youth tried to escape from the rear of the home and came toward three cops “in a threatening manner” with a large knife, Smith said.

One of three uniformed officers fired several times, killing Doerbecker, said Smith. At the time he confronted cops, he was carrying a knife with a 12-inch blade, said Smith.