Video shows a man repeatedly punching a man in the head and then choking him until he apparently lost consciousness.
No one appeared to help or intervene during the beating.
Police retweeted the video Monday to ask for any information on when the attack happened and the people involved.
The video was originally posted by an account that says the beating took place on a Manhattan-bound J train near the Kosciuszko Street station.
NYC Transit Interim President Sarah Feinberg said the video is horrifying and gut wrenching and she called on the city to provide additional policing:
"Beyond this single incident, assaults on transit workers, felony and misdemeanor assaults, robberies, and myriad 'quality of life' crimes are at unacceptable levels. I have called on the City and the NYPD to target additional law enforcement and mental health resources into the subway system, as soon as possible. While we have seen some additional resources from the City, sufficient resources have simply not been forthcoming. I call on the City again to provide the additional policing and mental health resources needed to address this crisis in our system. If we want this city to come back, if we want this economy to come back, the transit system must be safe, and it must feel safe - for our essential workers, for our children, and for every New Yorker from every neighborhood and every walk of life."
When asked if fellow passengers should have intervened, Feinberg said it's important to call for help as soon as possible.
"Do whatever you can to make sure that you're getting law enforcement getting police getting any kind of assistance you can including you know New York City Transit personnel, you can alert them at the station and they'll call for help as well," she said.
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