Clark said he did not know what sparked the animosity, but he described it as a case of "bullying that escalated to murder."
"For whatever reason, they had issues with each other," he said.
Detectives originally suspected that the gunman might have mistaken Haynes for someone he had tussled with earlier.
On Sept. 8, Haynes, who would have been a freshman at Roxborough High School this year, had gone to the bus stop around the corner from his home on North 28th Street.
He was standing there with two girls when Wilson rode past on a bike. The two exchanged words, Clark said.
Wilson, who lived with his grandmother, went to his house and returned with a handgun about 4:45 p.m., Clark said.
Clark said investigators were tracing the ownership of the gun.
After the shooting, a passing patrol officer rushed Haynes to Hahnemann University Hospital, where he died about 90 minutes later.
Haynes was buried yesterday after a standing-room-only service at the Choice G. Funeral Chapel.
"There were so many people there, you couldn't fit them in the mortuary at all," the Rev. Anthony Bowman said. "If there was one person there, there was 500."
Bowman, of New Hope Revival Center, had been mentoring Haynes after meeting him through his sister-in-law, a friend of Haynes' mother. He delivered a eulogy.
Bowman and the youth's family said Haynes was an average teenager who enjoyed typical pursuits for someone his age, particularly riding his mountain bike and dirt bike.
He is survived by his parents and five siblings.
"I'm at least glad they made an arrest," Bowman said. "Lord, have mercy. Another life wasted."
Contact staff writer Troy Graham at 215-854-2730 or firstname.lastname@example.org.