Analysts on Mr. Silber’s team who contacted the authorities about the threat were monitoring filters that scrape the internet for possible local attacks by employing search terms like “Jewish, New York, synagogue, kill, shoot and die,” said Mr. Silber, a former director of intelligence analysis with the New York City Police Department. The initiative was created after the 2018 Tree of Life synagogue attack in Pittsburgh, he said in an interview on Tuesday.

The filters search mainstream social media platforms like Twitter and Facebook, as well as other chat forums including 4Chan, 8chan, and Telegraph, he said.

The tweets on Friday morning, from an account linked to Mr. Brown, stood out from the online antisemitic chatter his analysts comb through daily because “they were talking about action” and included a time and day, Mr. Silber said.

Initially, the tweets indicated that perhaps the threat might occur on Long Island, so the consultants immediately alerted law enforcement authorities there. By early afternoon, the security team found additional online profiles seemingly linked to Mr. Brown that mentioned other threats, Mr. Silber said.

And by 2 p.m., it became apparent that the threat could be in New York City, at which point Mr. Silber’s team alerted city police officials.

“We basically told them that, ‘We know you get a lot of incoming, but you should pay attention to this,’” he said.

In a statement, Commissioner Sewell said that the department’s “exhaustive intelligence gathering led to the arrest.”

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