Alexis DeLaRosa, 18, was in a group hanging at the corner of North 6th and Clinton streets when Detectives Mustafa Dombayci and John Traynor spotted the gun's magazine sticking out of a fanny pack around his neck, Public Safety Director Jerry Speziale said.
As they got out of their car, DeLaRosa took off, Speziale said.
He ran down Jefferson Street, tossing the bag over a resident's fence, then crawled under a parked vehicle on North 7th Street, where he was taken into custody, the director said.
Detectives Suquan Gary and Levis Qirjako assisted, Speziale said.
The investigators recovered a 9mm semi-automatic High Point pistol and a 34-round high-capacity magazine, he said.
DeLaRosa was sent to the Passaic County Jail to await a first appearance in Central Judicial Processing Court on weapons-related charges, among other counts.
The gun was sent to the State Police lab to determine whether it may have been fired in any crimes.
Click here to follow Daily Voice Saddle Brook-Elmwood Park and receive free news updates.