On 2 October, Ryan Carson, 32, was killed near a bus station in Crown Heights, Brooklyn.

Before the unprovoked assault, the suspect was acting agitated.

Eric Adams, the mayor, promised that police would not rest until an arrest was made.

On a bench near a bus stop at about 04:00 local time (08:00 GMT), surveillance cameras captured footage of the suspect, dressed in dark clothing, Mr Carson, and an unidentified woman.

The suspect confronted Mr Carson and his companion after kicking over parked scooters and shouting obscenities at them, officials say.

In an effort to de-escalate the situation, Carson immediately placed himself between the man and his female companion, NYPD Chief of Detectives Joseph Kenny said on Wednesday.

A short time later, the suspect stabbed Mr Carson three times, including one blow that pierced his heart.

Mr Kenny explained that the man with the knife kicked Mr Carson in the chest, threatened to stab the woman companion, and spit in her face as Mr Carson lay dying on the sidewalk.

A woman believed to be the suspect’s girlfriend arrived soon after and apologized.

According to local media, police are withholding the identity of the suspect – an 18-year-old with a history of disorderly conduct – until they have enough probable cause to arrest him. On Wednesday, police released an image of the suspect and his girlfriend and asked for the public’s help in finding them.

For 13 years, Mr Carson lived in New York. His work with the New York Public Interest Research Group focused on waste policy spanned a decade.