With no gun recovered and without the testimony of E.J. and his girlfriend - who told police that she, too, saw Jones with the gun, but also failed to show up in court - the District Attorney's Office was forced to withdraw charges and was denied a chance keep Jones in prison.
The situation is as common as it is crippling for Philadelphia prosecutors.
"The district attorney and police commissioner have said, time and time again, we need the public's help," said Tasha Jamerson, a spokeswoman for the District Attorney's Office. "We're not able to prosecute crimes and get criminals off the streets if witnesses and victims do not come forward."
Jones was released from prison Aug. 8 after a Common Pleas Court judge sentenced him to house arrest for violating parole. Ten days later, Walker was walking to a bus stop after finishing his overnight shift when Jones allegedly shot him down.
Around noon Monday, as hundreds of family members, fellow police officers, and city officials mourned Walker at Deliverance Evangelistic Church in North Philadelphia, E.J., thin and goateed, sat bare-chested on his sofa while reruns of the sitcom Martin played on the television.
He explained his decision to not testify as simple street logic. "The repercussions aren't worth it," said E.J., who served six months in 2006 after a drug arrest. "People out here don't like that - snitching and going to court and all that."
The Inquirer is withholding the full names of E.J. and his girlfriend, Bernadette, because they were crime victims and their names are not part of the public court file.
E.J. said he did not know Jones at the time of the robbery at Knox and Hansberry Streets. He was new to the neighborhood and did not want to get a bad name.
"I didn't know who his people is," he said. "Anyone could have come into the courtroom and seen me."
E.J. picked up from the sofa a crumpled copy of the day's Philadelphia Daily News blaring the latest headlines on Walker's killing. He flipped the pages and put it back down.
He said he did not even know it was Jones who was arrested for Walker's killing. "I heard about the cop being shot," he said. "But I didn't know it was him. The name didn't ring a bell."
Bernadette, 22, sat holding their 5-month-old son. Their 6-year-old daughter sat in a chair in the corner watching the TV.
"I saw it on the news, and when they said his name, I thought it was him," Bernadette said.
Bernadette said she didn't go to court then because she was pregnant.
In their many efforts to try to get E.J. to court, the District Attorney's Office sent detectives to the apartment numerous times, including mornings he was scheduled to testify, but E.J. was never there. They contacted his family, sent letters, and called, sometimes finding the phone had been disconnected.
By the third hearing - prosecutors' last chance to move the case forward, Jamerson said - the District Attorney's Office obtained a bench warrant for E.J. to appear in court.
On Monday, E.J. claimed he had tried to make that final hearing but said he showed up late and missed it.
He wiped his face and swatted a fly. Then, he said he regretted his decision not to testify.
"I mean, I feel a little messed up," he said. "I should have went."
Contact Mike Newall at 215-854-2759 or firstname.lastname@example.org, or follow on Twitter @MikeNewall.