But Walker's lawyer, Thomas Fitzpatrick, said Monday that his client's crime paled in comparison to those of other officers implicated in a string of similar crimes.
"These are dangerous people who have committed significant criminal activity for some time," the lawyer said.
He would not say whether Walker had already testified before the grand jury but added that the former cop had given FBI agents incriminating information on "too many [officers] to count."
Assistant U.S. Attorney Anthony Wzorek would not discuss the investigation, saying only that Walker's "cooperation has been ongoing."
Walker's guilty plea comes a month after Police Commissioner Charles H. Ramsey first acknowledged the federal probe and said he had pulled at least four of its targets, all narcotics squad veterans, off of street duty.
The benched cops included Officers Thomas Liciardello, Perry Betts, Brian Reynolds and Michael Spicer.
Four others - Sgt. Joseph McCloskey, Lt. Robert Otto, and Officers John Speiser and Linwood Norman - worked closely with the officers under suspicion and were also reassigned. Norman was Walker's former partner.
Questions have dogged their work since at least 2012, when District Attorney Seth Williams said his office no longer trusted their testimony in drug cases, a decision that led to the dismissal of hundreds of prior arrests.
The city has since paid at least $777,000 to settle lawsuits claiming the officers framed suspects with false testimony and evidence. Dozens more remain on court dockets awaiting resolution.
None, aside from Walker, currently faces criminal charges.
Walker, 45, began cooperating with the investigation almost as soon as he was arrested last year in an FBI sting operation, his lawyer said.
An undercover informant caught the former officer on tape scheming to set up a South Philadelphia drug dealer by planting nearly 28 grams of cocaine in his car.
Walker later took his mark's keys, broke into his house and stole $15,000 and five pounds of marijuana.
He could face up to life imprisonment at a sentencing hearing scheduled for May 21.