No one is saying much.
"I believe somebody out there has seen it. We just haven't located them yet," said Sgt. Bob Wilkins of the Police Department's Homicide Task Force. "We're not getting too much cooperation out there."
As is typical in homicide cases in the city, police are offering a $20,000 reward for information leading to an arrest. Massey's killing was one of 228 so far this year.
A reserved young man whose family described him as a "gentle giant" - he was 6-foot-2 - Massey spent most of his teenage years in foster care, attending Marple Newtown High School, where he played football and basketball. He had hoped to attend college in January, his family said.
"With the nature of the crime, I'd imagine someone would speak up," said Corey Massey, Christian Massey's older brother. "But he wasn't really known around that area. Maybe people just don't want to get involved."
Police obtained two surveillance videos of the area but weren't able to get a clear view of the incident.
"It's like, if something happens, no one sees anything," said Carlton Hardy, whose girlfriend lives on the block. Both were at Hardy's home in New Jersey at the time of the shooting, he said, but police got a warrant for footage from his girlfriend's backyard security camera.
Police are analyzing footage from cameras in the area.
"I think a lot of neighbors are frightened," Hardy said.
As he lay shot in the alleyway, Massey managed to tell police what had happened to him. A would-be robber, he said, attempted to take his brand-new pair of Beats by Dre headphones. When he resisted, the man shot him and fled - leaving Massey's headphones and phone behind.
Then he told police, "I don't feel good." That was the last thing he said before he died.
"That's the only thing, basically, that we have," Wilkins said. "People are shutting down up there."
He said there had been other armed robberies in the area around Lebanon Avenue, but it was unclear whether they're connected to Massey's shooting.
The task force is asking anyone with any information on the incident to call police at 215-686-TIPS (-8477).
In the meantime, Massey's family is trying to "stay strong for each other," Corey Massey said.
"We're doing as best as we can at this point."