Walker, 40, was gunned down during an apparent robbery on Cecil B. Moore Avenue near 20th Street shortly after he finished working an overnight shift at the 22nd District's headquarters, at 17th Street and Montgomery Avenue.
A $118,000 reward was posted for the arrests of the killers, and Clark said tips began pouring in after surveillance footage was released that showed the heartless punks trailing ominously behind the veteran cop.
The reward money will likely go to one person who stepped forward with crucial information, Clark said.
Although the murder weapon has not yet been found, Clark said investigators are "very, very confident" that Jones was the triggerman.
Jones, 23, was arrested by investigators in a Southwest Philly public-housing complex on Wednesday, and reportedly admitted Thursday to some involvement in the crime.
His younger brother was questioned by detectives and offered "background" information, Clark said. The brother, whose name was not released, is not expected to face any charges.
McFarland - much like Jones - is no stranger to law enforcement.
According to court records, he was arrested on April 19 on drug charges, and was scheduled to face trial on July 31.
Records show, however, that prosecutors were not ready on that date, so the trial was rescheduled for Sept. 7.
McFarland was also arrested on May 8, according to court records, and charged with possessing marijuana. Records show that he completed the Small Amount of Marijuana diversion program.
Jones, meanwhile, has a criminal record that stretches back to his early adolescence.
Among the litany of crimes that Jones has been charged with in the past decade, an arrest in February for a gunpoint robbery triggered a violation of a probation sentence that he had been serving from a prior gun-possession case.
The probation violation led to a July 25 hearing in Common Pleas Court. "If I do get back out in society, I'm planning on getting my GED," Jones said, according to a court transcript.
Common Pleas Judge Susan Schulman noted that Jones had not complied with many of the requirements of his probation, and "went right back to running the streets that way you did before."
Schulman ordered Jones to be placed under house arrest with electronic monitoring for six months, enroll in a GED program, look for a job and submit to weekly drug tests.
"So, Mr. Jones," Schulman said, according to the transcript, "unless you enjoy spending your years behind bars, it's time to get it together."
Despite the judge's order, Jones was never placed under electronic monitoring, for reasons that are thus far unclear.
He submitted a urine sample Aug. 10 that tested positive for drugs, according to a law-enforcement source, and skipped an Aug. 15 meeting with his parole officer.
Ramsey declined Friday to discuss Jones' involvement with the court system.
"This time, perhaps, the system will put him away for the rest of his miserable life," he said.
- Staff writer Phillip Lucas
contributed to this report.
Contact David Gambacorta at email@example.com or 215-854-5994. Follow him on Twitter at @dgambacorta.