McFarland said that he and the other suspect whom police have in custody - Rafael Jones, 23, also of North Philly - intended to rob Walker, but the robbery went awry. McFarland said that Jones shot and killed Walker, Ramsey said.
Walker was off-duty at the time and in street clothes when he was shot on Cecil B. Moore Avenue near 20th Street.
Jones was captured Wednesday by police and U.S. Marshals and charged with Walker's murder Friday. In his statement, he had pinned the shooting on McFarland, Ramsey said. In his statement, Jones "claimed he only wanted to get a light" for a cigarette from Walker, Ramsey said, and didn't own up to trying to rob him.
Cops believe McFarland's version, that Jones - who has an extensive criminal record with arrests on gun, robbery and aggravated-assault charges - was the shooter.
Authorities have an arrest warrant out for McFarland on two store robberies that happened the same day in February 2011, said Tasha Jamerson, spokeswoman for the District Attorney's Office. He will be charged in those cases, she said.
McFarland is also expected to face charges in Walker's slaying.
As is customary when an officer is slain, Walker's handcuffs were to be placed on McFarland while he was being taken back to Philly. They had been placed on Jones when he was arrested.
McFarland faces a Sept. 7 trial on drug charges related to an April 19 arrest. He was also arrested in May for marijuana possession, and had completed the Small Amount of Marijuana Diversion Program.
Contact Julie Shaw at 215-854-2592 or firstname.lastname@example.org. Follow her on Twitter @julieshawphilly.