hackettstown middle school hallway

Hackettstown Middle School was searched this morning after a bullet is found outside.

(Express-Times File Photo)

police have revealed that a large-caliber handgun round was the

, prompting increased security.

Police responding to a call for a suspicious item found the bullet on school property, town police Sgt. Darren Tynan said.

The source of the bullet is under investigation. Officials put the school into a shelter-in-place mode in which students and staff were kept in class and allowed to leave only with police escort.

A New Jersey State Police K-9 team assisted with searching the four-story Washington Street school, including lockers and student backpacks, and no additional items were found, Tynan said.

"There was no weapon or anything located inside the school or around the school." he said.

The

last school year

.

Tynan said authorities are investigating whether today's incident is related to the North Warren discoveries or possibly a copycat situation.

"We don't know and that's what we're hoping that doesn't happen, that people think that this is something that's funny to do," he said, calling such incidents a waste of resources.

"We're always going to err on the side of caution no matter what it is because we always take everything seriously, especially around a school area," he said. "It will still be part of our investigation. We will look into every avenue to see if we can determine where it came from."

While teaching continued, students and staff were confined to classrooms for much of the morning. The incident began between 7 and 8 a.m. and everything was back to normal by about 11:30 a.m., Superintendent David Mango said on the town police Facebook page.