Detectives Suquan Gary, Mustafa Dombayci, John Traynor and Kenneth Kerwin found the weapon after stopping a late-model Kia whose driver blew a stop sign in the area of Rosa Parks Boulevard and Keen Street, Public Safety Director Jerry Speziale said.
The boy, who was sitting in the back seat, was carrying a Brand Herstal FN Five-seveN 5.7x28mm pistol reported stolen out of Mount Olive, he said.
"The weapon is extremely dangerous and fires high-velocity assault rifle ammunition," Speziale said.
The boy was being held in the Essex County Juvenile Detention Center pending a hearing in the Family Part of Superior Court in Paterson.
Other juveniles he was with were released to respective guardians, the director said.
The gun, meanwhile, was sent to New Jersey State Police for ballistics testing to determine whether it might have been fired in any crimes.
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