Moments after Sgt. Ritchard Blake shot and wounded a man who had confronted him amid a dispute over a woman they both knew, the sergeant patted down the man’s pockets as if looking for a weapon, surveillance footage shows.

There was nothing there: The man was unarmed.

But then Sergeant Blake, who was off-duty, pulled a sheath knife out of his back pocket and dropped it out of its covering beside the man, Thavone Santana, as he lay on a Brooklyn sidewalk, the video shows. After briefly pacing, Sergeant Blake picked it back up.

Those images of Sergeant Blake seemingly trying to plant a knife on Mr. Santana before thinking better of it are at the center of an intense investigation that has also focused attention on red flags that Sergeant Blake’s off-duty conduct had already raised.

Sergeant Blake, 40, had been arrested and charged with assaulting a woman in 2016 and was still on a form of administrative probation when he shot Mr. Santana, 21, early Thursday morning. Later on Thursday, the Police Department stripped Sergeant Blake of his gun and badge and placed him on modified duty.