It happened on William Floyd Parkway in Yaphank at midnight on Wednesday.
Police say the officer was directing traffic when a driver in a GMC Yukon stopped to ask him a question.
A Chevrolet pickup truck, driven by 38-year-old William Petersohn, slammed into the Yukon.
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The Yukon spun around, hit the officer and pinned him underneath it. Then, the pickup hit a marked police car.
The officer had emergency brain surgery and is on a ventilator.
Petersohn, the pickup truck driver, suffered serious injuries. He's charged with DWI.
The Yukon driver wasn't hurt.
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