Police say 38-year-old John Harvey, of Deer Park, was arguing with another driver in a Shop Rite parking lot in Massapequa.
It happened Sunday at around 10 a.m. at the location on Sunrise Highway.
The men were apparently arguing over a parking spot.
When a security guard tried to intervene, Harvey reportedly chased the guard with the large hunting knife.
The guard dropped his backpack as he backed away, and police say Harvey picked it up, got back in his car, and drove off with it.
When police arrived, Harvey was still in the parking lot. They placed him under arrest.
Harvey is now facing charges of first-degree robbery, second-degree menacing, and criminal possession of a weapon.
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