Police have released surveillance footage of a man suspected of shoving a stranger off a subway platform to his death in New York City as they hunt for the unidentified assailant.

Wai Kuen Kwok, 61, was standing with his wife Sunday afternoon at a station in the Bronx. Authorities said that without warning, a man pushed him onto the tracks as a "D" train was entering the station. His wife, who was not injured, told police the act was unprovoked.

Authorities said the suspect fled the station, hopped onto a bus and took it a few stops.

Surveillance footage shows the man walking calmly from the subway station. Later footage shows him getting off a bus, heading into a convenience store and then emerging smoking a cigarette.

The suspect is balding and was dressed in a leather jacket, dark pants and white sneakers.

Kwok, a father of two who was from the Bronx and worked in a factory in Brooklyn, was standing with his wife off the platform at the Grand Concourse and East 167th Street station in the Highbridge neighborhood. He was struck by the train at around 8:40 a.m.  and pronounced dead at the scene. Police immediately classified his death a homicide.

The motorman, who reportedly witnessed the push, slammed on his brakes, but it was too late, a transit source said. The first car of the train struck Kwok, and several of the cars then rolled over him.

Kwok's son told the New York Post his father -- who worked long hours in a kitchenware factory in Brooklyn -- was on his way to Chinatown with his wife to do their weekly grocery shopping

"He wanted us to be successful," his younger son told the newspaper. "His plan was to work and get his sons through school."

Police were seen dusting the bus for fingerprints Sunday night, according to the Post. 

There have been three other incidents in recent years that involved a person being pushed onto the tracks.

In April 2013, a train ran over a man desperately clawing at a Manhattan subway platform after being pushed onto the tracks by a homeless suspect with whom he'd been arguing.

In December 2012, another homeless man was arrested for pushing a Queens straphanger in front of a Times Square train that fatally crushed him.

And later the same month, a mumbling woman pushed a man to his death in front of a subway train in Queens.

The Associated Press contributed to this report.