Four people were wounded at a Brooklyn subway station Sunday when police officers shot a man threatening them with a knife, and inadvertently sprayed bullets that hit passengers, authorities said.

The people struck by gunfire included two innocent bystanders, one of the officers and the man with the blade, who the police initially confronted because he hadn't paid his fare, officials said.

One of the passengers, a 49-year-old man, was hospitalized in critical condition after a bullet passed into an adjoining subway car and struck his head.

A video posted by a passenger showed upset passengers fleeing, police running to help the injured and the wounded officer suddenly realising he had also been struck by a bullet.

Interim Police Commissioner Thomas Donlan, who was only appointed to his position Friday, promised a thorough investigation, but cast blame for the incident on the man accused of brandishing the knife.

“We will be working through the timeline of today, but make no mistake, the events that occurred on the Sutter Avenue station platform are the results of an armed perpetrator who was confronted by our officers doing the job we asked them to do," Donlon said.

The shooting happened a little after 3 pm when two officers followed a man up the station steps to an elevated platform after seeing him enter without paying, Chief of Department Jeffrey Maddrey said.

The officers told the man to stop, but he refused, muttering “I'm going to kill you if you don't stop following me,” Maddrey said. In the course of the encounter, the officers noticed the man had a knife, Maddrey said.

They followed him onto a train that had pulled into the station and fired two Tasers, but neither incapacitated the man, Maddrey said.

Maddrey said the man was advancing on the officers with the knife when both officers fired multiple rounds.

The 37-year-old man was hit several times. He was hospitalised in stable condition. The video taken by a passenger shows the officers rendering first aid, before one of them realised that he, too, had been hit by a bullet.

“While they're working on the male, they're become aware that other people are hit by fire, by gunfire as well,” Maddrey said.

Besides the passenger struck in the head, a 26-year-old woman suffered a graze wound.

The wounded police officer had a bullet enter his torso under his armpit and lodge in his back but was expected to make a full recovery.

Mayor Eric Adams, a Democrat, visited the wounded officer in the hospital Sunday.

Adams described the man who evaded the subway fare as a “career criminal,” saying he had over 20 arrests. Maddrey said the man had a history of mental illness.

Video footage of the shooting was not immediately released Sunday. The NYPD did release a cropped image they said was of the man holding the knife, a blade about the width of the person's palm.

“I'm especially concerned with bystanders, people who are just trying to get where they're going being the victims—harmed in this situation,” Metropolitan Transportation Authority Janno Lieber said.

The subway station serves the L line in the neighbourhood of Brownsville. Lieber said that there are cameras inside the the train, on the platform and at the entrance.

In 2019, NYPD officers accidentally shot and killed two fellow officers while confronting crime suspects in separate on-duty incidents.