The video was posted to Facebook, a video of teens laughing as one of them goes up to an unarmed 58-year-old man sitting at the Aronimink trolley stop in Upper Darby and punches him square in the jaw.

The punch is so hard you can hear it.

"It was a violent, vicious, cowardly, punk act," said Upper Darby police Superintendent Michael Chitwood. "He's lucky he didn't kill him."

Chitwood said the unprovoked violence was part of the "knockout game."

"They were conspiring to go in and do the knockout game thing on this guy," he said. "It fits the profile: They plan it, they videotape it and they Facebook it. The whole thing, and they did it all."

Police first became aware of the incident, which occurred shortly after noon last Wednesday, when the Facebook video was posted by one of the four boys involved and was brought to their attention by concerned citizens who saw it online.

In the video, one of the teens can be seen approaching the man inside the trolley stop. The victim says: "I ain't got no money," and the teen tells the man: "I don't want none of your money." After that, the victim asks the teen "What's up?"

"What are you trying to do?" the teen asks.

"I'm not trying to do nothing, just sitting here," the man says.

That's when the cameraman says, what sounds like, "Johnny about to go, Johnny about to go," and the kid cold cocks the man square in the jaw.

Working backwards from the video, on Thursday police traced the location of the assault to the Aronimink trolley stop at Burmont Road and Morgan Aveune. When officers arrived, they found, sitting at the stop, the same man who had been attacked there the day before.

Chitwood described the 58-year-old victim as "semi-homeless" and the trolley stop is just where he "hangs out."

"He's got no place else to go," Chitwood said of the man.

Authorities were able to grab additional video from a nearby bank surveillance camera that showed four teens were involved in the incident. Chitwood said the department has been inundated with calls about the identity of the doer.

"So he should know we are coming and when we break his door down and take him out in cuffs, he'll know we got him," he said. "Thank God we're not dealing with rocket scientists."

Anyone with information on the identity of the teens is asked to call Upper Darby police.