A 32-year-old man who shot a second man in the head during an altercation on a moving A train Thursday evening appears to have acted in self-defense and will not be criminally charged for now, the Brooklyn district attorney said on Friday.

The shooting, which followed a frightening, chaotic confrontation on a crowded subway car during the evening rush, left the second man, 36, in critical but stable condition. The gun he was shot with was one he brought onto the train and brandished during the altercation, the police said.

Oren Yaniv, a spokesman for the district attorney, Eric Gonzalez, described the shooting as “shocking and deeply upsetting.”

“The investigation into this tragic incident is ongoing,” Mr. Yaniv said in a statement, “but, at this stage, evidence of self-defense precludes us from filing any criminal charges against the shooter.”

The confrontation that preceded the shooting involved the 36-year-old man who wound up being shot and the 32-year-old, whom the older man was harassing as the northbound A train pulled into Hoyt-Schermerhorn station in Downtown Brooklyn around 4:45 p.m., the police said.

The two men did not know each other, the police said, and it was not clear how the altercation began. But cellphone video of the episode that captured the minutes leading up to the shooting shows the eventual victim screaming threats and racist remarks at the second man.

Officials had not released the name of either man as of Friday morning and were also searching for a woman who appeared to have stabbed the 36-year-old man before the shooting occurred.

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 the two square off.

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

The older man, blood seeping through his white T-shirt, pulls a gun from his coat and confronts both the younger man and the woman, who have moved to the far end of the subway car.

As other panicked riders rush to the other end of the car, the video shifts away from the fight. Gunshots and screaming can be heard.

At a news conference on Friday, Jeffrey Maddrey, the Police Department’s chief of department, said that the 36-year-old had pulled out a gun during the dispute and that the younger man had taken it and shot the older man.

The police released a video at the news conference that showed the 36-year-old man entering a subway station without paying a fare; it was not clear which station. The police said on Thursday that the altercation had started after the 32-year-old man got on the A train at the Nostrand Avenue station.

“Evidence is still unfolding,” Chief Maddrey said on Friday, adding, “Oftentimes, we see people enter into the subway station looking to cause harm, and they never pay the fare.”

Hermann Walz, a former state prosecutor and adjunct professor at John Jay College of Criminal Justice, noted in an interview on Friday before Mr. Gonzalez announced his decision that it was ultimately up to the district attorney, based on the evidence, whether to file charges.

In June, a grand jury in Brooklyn voted not to indict a man who had fatally stabbed a passenger on a subway train. The man, Jordan Williams, 20, of Queens, stabbed Devictor Ouedraogo, 36, on a northbound J train, the police said.

Mr. Ouedraogo had gotten into a dispute with other passengers, including Mr. Williams’s girlfriend, according to law enforcement officials. Witnesses said Mr. Ouedraogo threw the first punch, an official said.

In an interview with NY1 Friday morning, Janno Lieber, the chief executive of the Metropolitan Transportation Authority, the state agency that operates the subway, called the episode “harrowing.”

“When you’re in that situation, in a subway car and something that bad is going on and you’re fleeing for your own safety, that’s just the nightmare for all of us at the M.T.A.,” he said. “That’s a nightmare for New Yorkers.”

The shooting inside the Hoyt-Schermerhorn station, which is served by the A, C and G trains, occurred a week after Gov. Kathy Hochul deployed hundreds of National Guard members and state troopers to check bags in the subway system. Ms. Hochul described the move as an effort to deter crime and ease riders’ fears. The altercation also came amid growing complaints that the M.T.A. has not done enough to guarantee worker safety.

A string of violent episodes in recent years has left some subway riders on edge.

In 2022, Michelle Alyssa Go was on a subway platform in Times Square when a 61-year-old man pushed her from behind, shoving her to her death in front of a southbound train. Months later, there was a mass shooting on the R train in Brooklyn.

More recently, a 45-year-old crossing guard was fatally shot on a train in Brooklyn in January after intervening in a fight, and a 35-year-old man was killed and five other people were wounded after an argument broke out among teenagers on a train in the Bronx last month.

In response, officials introduced several anti-crime initiatives in the subway, including adding more police officers and increasing overtime pay for them, and began removing severely mentally ill homeless people from the system.

Surveys by the M.T.A. consistently show that some riders perceive the subway as dangerous, even when data does not necessarily confirm that view.

In a post on the social media site X late Thursday, Jumaane Williams, New York City’s public advocate, said he had seen videos of the “terrifying moments” for straphangers and he implored Ms. Hochul to do more to increase subway safety.

Officers at the Hoyt-Schermerhorn station, which has a police outpost, heard gunshots as the train arrived there on Thursday and responded immediately, said Michael Kemper, the Police Department’s, chief of transit, at a news conference that day.

According to an internal police report, the first officer on the scene was engaged in a transit inspection when he heard four gunshots as the train pulled into the station. The officer took cover and saw a man run out of a subway car with blood on his face, at which point officers stopped him at gunpoint and placed him on his stomach, the report said.

The officers saw a man lying under the seats of a subway car, not moving and surrounded by blood, the report said. The officers also found a gun, a 38-caliber Ruger, a few feet away.

The older man had two stab wounds to his lower back and “penetrating” wounds to the right side of his neck, the left side of his chest, his right temple and just below his right eye, which was ruptured, the report said.

Another video posted on X shows the younger man splayed on his stomach on platform steps. As he is led away in handcuffs, the 36-year-old man can be seen lying on his back through the subway car windows. He appears to move his head very slightly as officers begin tending to him.

Moments later, the man can be seen lying on his side and facing the window, a red stain visible on his back, as officers in white gloves examine him.

Chief Maddrey said the public response had been “incredible,” with videos recorded by people on the train giving “us additional evidence that we needed to piece together questions that we had.”

Chelsia Rose Marcius contributed reporting.