PATRICK WALTERS, The Associated Press
PHILADELPHIA (AP) -- Before planting an electronic bug as part of an investigation, authorities with the FBI or another federal law enforcement agency must go through a secret application process and convince a judge all other attempts to get answers have failed.
With a judge's approval, federal law allows investigators to surreptitiously enter a premises when setting up listening devices.
Federal law enforcement officials, who spoke on condition of anonymity, have acknowledged that the FBI was responsible for bugs found Tuesday in the office of Philadelphia Mayor John F. Street, but refused to provide details about the nature of the probe.
There are typically about 700 to 900 requests annually for a court-issued wiretap warrant, according to law enforcement experts.
An FBI agent in a local field office must first submit an affidavit, setting forth under oath the reasons why an office needs to be bugged, said Nicholas Gess, an associate deputy attorney general for the Clinton administration.
Then an assistant U.S. attorney writes an application to be approved by a U.S. attorney and an FBI special agent before being sent to the FBI headquarters and the Office of Enforcement Operations in Washington, D.C. Once it gets to the Justice Department, only eight people are empowered to approve the request -- the attorney general, the deputy attorney general, the assistant attorney general for the criminal division or one of five deputies within the division, Gess said.
After it's approved, the request goes to a federal judge, who can approve it, reject it, or send it back for changes.
The government needs to show probable cause that a crime has been committed and also prove to the judge that bugs are "investigative technique of last resort," said David Sobel, general counsel with Washington-based Electronic Privacy Information Center.
"The point is that electronic surveillance is seen as a particularly intrusive investigative technique," Sobel said.
A judge can also authorize "surreptitious entry" in the order if it's determined that the bug cannot be placed any other way, Gess said.
While government bugging isn't particularly common, it most frequently comes up in cases of organized crime and public corruption investigations, said J. Clayton Undercofler, a first assistant U.S. attorney in Philadelphia from 1972 to 1976.
Street has said that he was unaware the bugs were in his office, something that experts said wouldn't be unusual, whether he was the target of an investigation or not.
"I would be very, very surprised if the FBI told anybody in the mayor's office," Gess said.