Dion Duncan, 41, was pulled over near the intersection of E. 41st Street and 18th Avenue for making an illegal U-turn, Passaic County Sheriff Richard Berdnik said Monday.
He appeared "extremely nervous" and moved toward the glovebox, Berdnik said.
Sheriff's Officers Joseph Royce and Chelsea Parisi retrieved a Starfire 9mm handgun that had a round in the chamber and 11 rounds in the magazine, the sheriff said.
Duncan was ordered held. Bail information wasn't immediately available.
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