McCloskey says he was just playing a joke on a friend.
But what the 19-year-old calls a prank, police say involved a homemade bomb on a local street.
The improvised explosive device set up in a backpack containing nails, gunpowder and electrical wires.
"It was very dangerous. The bomb squad actually had to detonate the device. It was made up of gunpowder, wires, and nails so that it would have a stratenal type of effect," explained District Attorney Dan Donovan.
Police picked up McCloskey just after 9 Saturday night in the Stapleton section of Staten Island.
A savy officer spotted the teen pacing, sweating, and yelling into his cell phone at the corner of Broad and Canal Streets.
That officer asked the Staten Island man about a bulge under his jacket.
McCloskey allegedly said, "I made it. I'll show it to you. I'm going to prank my friend with it."
"This was no prank. This device could have caused injury and death a lot a lot of people," said Donovan.
Detectives say McCloskey learned how to make the IED on the internet.
No answered the door at McCloskey's Burton Avenue home, but neighbors described a troubled teen from a solid family.
"He's not all there. I think so. I think he has a problem," said one neighbor.
19-year-old John McCloskey faces felony charges for the homemade backpack weapon.