Detective Thomas Gaul, the lead investigator, said he was not authorized to speak about the facts of the case but was able to say of the victim: "It was not the result of him being involved in anything illegal. His killing was a senseless, cowardly act."
Jett had never been in trouble with the law, police said. A cheerful jokester outside the ring but dedicated inside it, Jett had chosen the gym over the streets, his family said.
Davis, who lived a few blocks away from Jett's mother, had sold Jett a late-model junker, said Sgt. Bob Wilkins of the Homicide Task Force, which handled the case with help from officers of the 12th District and Criminal Intelligence Unit.
When the car broke down a few days later, Davis refused to refund Jett's money even after the teen had returned the car, Wilkins said.
The argument got physical, and Jett bested Davis, who was seven years older. Embarrassed, Davis got a gun.
Jett, who lived in Northeast Philadelphia with his father, never saw Davis coming, Wilkins said.
Moments after he was shot, his mother, Lychelle Wright, held him on the street. He died 20 minutes later at the Hospital of the University of Pennsylvania.
Jett was buried Monday. His father, Alphonso Jett, a bicycle-patrol officer in Center City's Ninth District, told the large group of friends and family at Pinn Memorial Baptist Church that he had not only lost his only son, but his best friend.
After the funeral, Jett recalled how he had been surprised when his 12-year-old son insisted on taking up boxing. He took his son to Upper Darby Boxing, and the slender youngster took to the violent sport.
"I never thought it would go as far as it did," he said.
Jahmeer Jett relied on speed and smarts rather than brawn, and Alphonso Jett found himself accompanying his son to boxing tournaments, including the Golden Gloves Nationals in Nevada in 2011. The teenager lost in the finals, but he and his father had dinner with the famed trainer Roger Mayweather.
"He smiled outside the ring, but when he got in the ring, he took care of business," Jett said.
Hours before he was killed, Jett texted his trainer, Eugene "Chip" Hart Jr., a question.
The junior welterweight had notched 35 victories against five defeats as an amateur, and had earned a state Golden Gloves championship. Now, although he had a day job as a dietitian in a nursing home, Jett was mulling a pro career.
"Do you honestly think I have what it takes to make it in boxing?" Jett asked.
"You have everything it takes - you just have to find if you have the passion to do what it takes," Hart wrote back.
There was no doubt in Jett's reply:
"I know I have what it takes. And I am ready."
On Monday, Hart said that he had a plan for Jett: three more amateur matches, then go pro. His next bout was scheduled for Feb. 14.
"He had unbelievable potential," Hart said. "At the age of 15 or 16, he sparred professional fighters and dominated some of those guys."
But beside all his potential in the ring, Hart said, he would be remembered most for the person he was outside it.
"He will always be remembered for his smile," he said. "It's hard to believe something like this could happen to such a beautiful person."