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.
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 coldcocks the man square in the jaw.
On Thursday, police traced the location of the assault to the Aronimink trolley stop at Burmont Road and Morgan Avenue. When officers arrived, they found, sitting at the stop, the same man who had been attacked there the day before.
Chitwood described the victim as "semi-homeless" and said 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 that 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 at 610-734-7684.
On Twitter: @FarFarrAway