Butler police arrested a 13-year-old boy after he showed another boy a handgun during an altercation Monday night at the YMCA on North Washington Street.

No one was injured and police seized the pistol, a Ruger 9 mm that was loaded with a magazine containing five rounds.

The gun belongs to the boy's grandfather, with whom he lives in Summit Township, according to Deputy Police Chief David Adam.

Police did not identify the boy, whom they charged Tuesday in Butler County juvenile court with possession of a handgun by a person under 18, carrying a firearm without a concealed carry license, and terroristic threats.

Investigators believe the incident happened about 7 p.m. in the gymnasium, where the alleged victim, a 14-year-old Butler boy, was horsing around with another boy.

Nearby were at least two other boys, including the defendant. At some point, Adam said, the 13-year-old warned the victim not to touch him as the older teen was playing around.

When the victim touched the defendant's knee, the 13-year-old made another warning: “You don't want to touch me. I have something to protect me,” according to a police report.

The boy opened his book bag and allegedly showed the other teen a gun that was in it. The 14-year-old was the only one to see the gun.

The victim subsequently went to a YMCA employee and reported what happened. The employee notified authorities.

Both boys were YMCA members, according to a statement by the Butler YMCA.

YMCA staff reviewed surveillance footage, which showed five people, including the teen wearing the backpack, in proximity, though the footage did not show any physical altercation. The facility's surveillance was updated this year with additional cameras and a broader view of public areas.

“We are thankful that the other teenager reported what he saw in the gym,“ says Dave Hilliard, president/CEO in the statement. “Our staff followed our established procedures and immediately called the police, who responded to the call within minutes.”

Police got there and found the defendant and a friend walking down the steps from the gym, Adam said. When officers asked the boy what was in the book bag, he acknowledged he had a gun.

A search of the bag turned up the pistol, which was in one of the bag's pockets. While the gun was loaded with a magazine, Adam said there was no round in the chamber.

When asked why he had the gun, the boy said it was for self-defense.

“He stated he was worried about people trying to fight him,” Adam said, “and he believed he needed a gun to protect him.”

Police took the boy to the station and notified juvenile probation authorities. He was eventually released to the custody of his grandfather.