A 36-year-old man was in critical but stable condition on Friday morning after he was shot in the head onboard a northbound A train as it pulled into the Hoyt-Schermerhorn subway station in Downtown Brooklyn during Thursday’s evening rush hour, according to authorities.

The shooting was the culmination of a frightening confrontation on a crowded subway car, in which an armed man who had been menacing another passenger ended up being shot with his own gun, the police said.

The two men did not know each other, the police said, and it is not clear how their confrontation began. But video of the episode, which captures the moments leading up to the shooting, shows the eventual shooting victim, identified by authorities as a 36-year-old man, screaming threats and racist remarks at the other man, 32.

Officials have not released either man’s name. On Friday morning, they were also searching for a woman who appeared to have stabbed the older man.

In the video, the 36-year-old man stands over the 32-year-old, walking back and forth in front of him and repeatedly threatening him, saying “I will beat you up.” Eventually, the 32-year-old man stands up, and they square up to fight.

The older man wrestles the younger man into a seat and stands over him, pummeling him. At that moment, a woman rushes up and appears to stab the older man in the lower back. It is not clear if the woman knew the younger man before the encounter.