Story highlights NYPD makes two additional arrests in connection with alleged threats

Police have monitored social media for such activity

This follows the weekend killing of two officers in Brooklyn

New York (CNN) Nine men have been arrested in connection with alleged threats made toward New York police since Saturday afternoon, when two NYPD officers were shot in the head from behind while sitting in their patrol car, authorities said.

Police said this week that they have assessed hundreds of online postings and calls to 911 and pressed the public to continue to report any suspicious activity.

"All threats against members of the NYPD are taken seriously and are investigated immediately to determine the credibility and origin of the information," authorities said Wednesday.

The two latest arrests were announced Friday.

In one case, a man walked into the 28th precinct, approached a desk and harassed and threatened officers before being apprehended, police said. He faces charges of assault on a police officer, resisting arrest, obstructing government administration, criminal trespassing, disorderly conduct and harassment.

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