Four of the officers - Perry Betts, Michael Spicer, Thomas Liciardello, and Brian Reynolds - had been moved from the Narcotics Field Unit in December 2012 and placed in lower-profile departmental jobs.
The U.S. Attorney's Office in Philadelphia has repeatedly declined to confirm or deny the existence of the probe or the grand jury.
The 2012 action came after the District Attorney's Office informed Ramsey that prosecutors no longer wanted to call the officers to testify in drug cases. The District Attorney's Office was concerned about the officers' credibility, sources said.
Attorneys for those officers could not be reached for comment.
The two others who were pulled from the street - Sgt. Joseph McCloskey and Officer Linwood Norman - worked closely with those narcotics officers for many years.
No charges have been filed against the six officers, but all were required to surrender their guns, a department spokeswoman said. She said all were now assigned to desk duty, five at Police Headquarters and one in Traffic Court.
Ramsey also said a narcotics officer, Jeffrey Walker, was in custody. Walker was arrested on federal corruption charges in May in a sting operation by FBI agents working with police investigators. They recorded Walker bragging about how easy it was to rob drug dealers.
Ramsey gave no details on the investigation, other than saying it involved a federal grand jury. He offered no specifics, since federal law requires secrecy of the grand jury's work.
Since the December 2012 transfer of the narcotics officers, Philadelphia courts have dismissed hundred of drug arrests.
The city has been hit with about 40 federal civil rights lawsuits involving the officers. Many of the suits allege that the officers framed suspect with false testimony and evidence.
Public defender Bradley S. Bridge, who has been involved in many of the convictions of people arrested by the drug unit, applauded Ramsey's actions.
"That these officers have now been taken off the street is significant," Bridge said. "Substantial questions have arisen regarding the propriety of their actions while making arrests. Until those questions have been answered satisfactorily, the most prudent course is to prevent them from making more questionable arrests."