A suspect reportedly has been taken into custody Monday following the murder of a man on a New York City subway platform a day earlier in what is being investigated by police as a "gang-related" attack, possibly involving an MS-13 member.

The identity of the suspect is unclear, but a police source that spoke to the New York Post said the individual is currently being questioned by police at a precinct house.

“We have a person in custody,” the source told the newspaper. “Motive is gang-related.”

Law enforcement sources told NY1 earlier today that the shooter belongs to MS-13 and the victim was a member of the 18th Street gang, one of MS-13's rivals. An interactive map created by the New York Daily News shows both gangs have been active in the area where the killing happened.

Police have been looking for two persons of interest seen in a graphic video of the shooting that has been circulating on social media. Police also have released an image of one of the persons of interest -- the suspected shooter -- fleeing the 90th Street-Elmhurst Avenue subway station in the Queens borough. It was not immediately clear Monday if that person is the one in custody.

Police say the incident happened around noon Sunday when the victim and two men started arguing on a train and the dispute spilled out onto the platform.

It's uncertain what the disagreement was about – but the video first shows the men wrestling with the victim on the ground amid the cries of terrified passengers.

The shooter then breaks free, pulls out a small handgun and is heard firing at least six shots at the victim, sending those on the platform running for their lives. The footage ends with the shooter escaping down the steps of the now-empty platform as the victim lays motionless nearby.

“This incident is beyond horrifying," MTA spokesman Max Young said. "While our system and the city are safer than ever before, we have zero tolerance for any violence on our subways and buses, and the safety of our customers and employees will always be our number one priority.”

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The victim died at the scene and the New York Times reported it was the first killing on the New York City Subway system in more than a year.