Metro Transit Police arrested a teen today after a viral Twitter video showed him allegedly attacking a Metro passenger.

In the video, originally posted by Twitter user @DeefrmUPT, two teenagers on a Green Line Metro train in Prince George’s County egg on their friend to attack an unsuspecting passenger, a young man who looks to be in his 20s.

“You don’t do it, you’re a bitch,” says one person in the video. “It’s just straight like that.”

One teen slaps the victim in the back of the head. When the victim responds by asking “what the fuck is wrong with you,” his attacker and another teen close in on him, urging him to “say something else.” The attack happened around the middle of the day on Monday.

The Twitter user who posted the video eventually made his Twitter account private. The video went viral among Donald Trump supporters on Twitter and white nationalists on the website 4chan, apparently thanks to the racial difference between the attackers (black) and victim (white). On the 4chan thread about the video, users organized to send tips about the case to police and “dox”—or reveal personal information about—the teens in the video.

Metro spokeswoman Morgan Dye says the juvenile arrested in the case will be charged with 2nd degree assault. The victim reported the attack to Metro Transit Police, according to Dye, and declined medical help.

“Clearly the investigation rapidly advanced after the video surfaced,” Dye says.