A New York City subway robbery attempt fell apart Tuesday night when an unidentified bystander pulled out a gun and fired a shot inside one of Manhattan's major subway stations, according to police.

He overheard the would-be mugger demand a woman give him money and threaten to steal her purse, according to authorities. Then he allegedly fired a single shot to save the 40-year-old stranger at the busy 49th Street subway station, near Rockefeller Center, around 9:15 p.m. Tuesday.

The NYPD is asking for the public's help identifying him in connection with what authorities are describing as a "reckless endangerment incident."

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Subway vigilante suspect leaves through turnstile

The New York City Police Department is asking the public's assistance identifying the whereabouts of a man who they say thwarted a robbery attempt by firing off a gun inside a Manhattan subway station Tuesday evening. (NYPD)

The robbery suspect, 49-year-old Matthew Roesch, has already been arrested, police said. He has one prior arrest – but it came in September.

But police are still looking for the unidentified man who appeared on surveillance video thwarting the attack on the northbound platform Tuesday night, New York authorities told Fox News Digital.

The man left on foot, and his whereabouts were unknown as of Wednesday morning.

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Subway vigilante suspect shown carrying shopping bags

The incident happened around 9:15 p.m. on the northbound platform of the NRW subway station at 49th Street, near Rockefeller Center in midtown Manhattan Tuesday. (NYPD)

Authorities described the man they are looking for as a light-skinned man between 35 and 45 years old. He is around 5 feet, 9 inches tall and about 230 pounds. He was wearing a green T-shirt, black shorts, black sneakers and white socks. He was carrying a green backpack and also a green reusable shopping bag from B&H, a popular camera store.

No one was injured during the incident, according to police.

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Ralph Cilento, a retired lieutenant commander of detectives at the NYPD and adjunct professor of police science at John Jay College of Criminal Justice, had predicted a rise in vigilanteism last year after another New York man shot and killed a robbery suspect on a Queens sidewalk and a belligerent straphanger died after a physical encounter in which a U.S. Marine veteran placed him in a chokehold. 

New York has a "duty to retreat" law in which physical force is allowed in self-defense or the defense of others only if the subject feels that they cannot retreat and life is in danger.

While city crime stats show slight declines now after spiking during the coronavirus pandemic, he said even a small decline is a positive sign but that rank-and-file officers have been jarred by frequent leadership changes in the upper echelons of the NYPD, which coincides with public unease.

"People of New York City are armed now," Cilento told Fox News Digital. "It’s not just guns… people are walking around with knives on them now, working women in midtown, they’re carrying Tasers. People have to feel that their police department is going to uphold the social order."

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The NYPD has visibly increased its presence within the city's subway system. Earlier this week, officers were there to rescue a man who fell on to the tracks in Queens. 

"The increase in patrols in subways is definitely a good thing, nobody would say otherwise," he told Fox News Digital. "There's definitely tangible results."

But people are still worried. Just last month, a repeat offender shoved a woman off a nearby subway platform into a moving train, nearly killing her. 

Police statistics show that transit crimes have fallen by 3.6% so far this year compared to last – but are up 29% over 2021.

"It goes down 3.6%, you can't turn up your nose from that," Cilento said. "But it's down from such a high that the overall trend has to carry down – the sample size is not big enough to take a victory lap."