Public schools in Montclair, N.J., were closed on Monday after a teacher at an elementary school threatened its principal on social media, the police said. Several hours later, the school district announced that the potential threat had been “neutralized.”

The teacher, Amir Doctry, was arrested in Philadelphia on Monday morning and charged with making terroristic threats, a spokesman for the Montclair Police Department, Lt. Terence Turner, said.

Mr. Doctry, a teacher at Northeast Elementary School, was put on administrative leave last week when he started showing signs of “erratic behavior,” Lieutenant Turner said. Mr. Doctry was in a “manic state” when he was apprehended and taken to a Philadelphia hospital for a psychological evaluation, Lieutenant Turner added.

The Montclair Police Department said it was increasing patrols around all schools out of an abundance of caution. The department added that there was “no further danger” to Montclair, a township in Essex County, N.J., that is about 20 miles from New York City by car.

Mr. Doctry is listed as a “long-term teacher” on the school district’s website, but he was not listed as a staff member in the Northeast Elementary School’s directory as of Monday afternoon.

A LinkedIn profile for a user with his name says that he owns a business geared toward creating virtual-reality lessons for students and that he previously taught sixth-graders in Montclair and students in Newark.

Not long after the school district announced the school closure on Monday, parents began to speculate that it was connected to a YouTube video featuring Mr. Doctry. Lieutenant Turner confirmed the connection in an interview.

In the video, Mr. Doctry can be seen holding up a letterman jacket and saying that he believed he would be promoted to school superintendent and planned to get the title “superintendent” stitched onto the jacket once that happened.

“That is how confident I am that I am going to be the new superintendent of Montclair Public Schools,” he said.

He later added: “We need change. That is the whole point,” before making a series of remarks about artificial intelligence.

The video’s lengthy caption contained a string of profane, political and racist assertions. It read, in part: “Joe must die school shooting Montclair school shooting kills Dr. Joe Racsim Racist Joe is dead. He dies tonight.”

The school’s principal is listed as Dr. Joseph Putrino. Lieutenant Turner confirmed that Dr. Putrino, who did not respond to a request for comment, was the target of the threat.

Damen Cooper, the interim superintendent of Montclair Public Schools, had said in an email to parents early Monday that the district became aware of the potential threat late Sunday night and had decided to close schools to be safe.

“I understand that this last-minute closure may cause inconvenience, but the safety of our students, staff, and entire school community is our highest priority,” the email said.

Several hours later, Mr. Cooper advised the community that the potential threat had been “neutralized,” thanks to a collaboration with the Police Department.

Matthew Frankel, who has two children in Montclair public schools, one of whom was tutored by Mr. Doctry during the pandemic, said the news had been a “shock to the system.”

“There was transparency within our community, and I think in a moment of crisis, that’s what we want as parents,” he said. “In terms of how we feel about this teacher, and even speculating on his mental state, I think the most important thing that we can do is to let the authorities do their job.”