The FBI says it's easy to complain about leaks, but hard to find the leakers.

U.S. intelligence agencies have made 183 complaints about unauthorized disclosure of classified information in the past five years, but the FBI has identified just 14 suspected leakers, none of whom has been prosecuted, according to data assembled by the Justice Department.

Despite getting 183 complaints, the FBI opened just 26 leak investigations, often because referrals from intelligence agencies lacked sufficient information to proceed, FBI Director Robert Mueller told Senate Judiciary Committee chairman Patrick Leahy, D-Vt.

The starting point for a leak investigation is identifying everyone with authorized access to the information, and the most typical information gap is a failure to identify them all.

Even when investigators find a suspect, the leaker often cannot be identified beyond a reasonable doubt, as is required for a successful prosecution.

"Because indictments in media leak cases are so difficult to obtain, administrative action may be more suitable and may provide a better deterrent to leaks of classified information," the FBI director said in April in response to questions from Senate Judiciary Committee member Sheldon Whitehouse, D-R.I. Whitehouse had asked the questions following a Senate hearing last September.

Steven Aftergood, head of the Project on Government Secrecy, first reported on the leak probe data on Monday.

The Justice Department's national security division oversees the work on all leak probes and the FBI has substantial discretion.

Even without a referral from another agency, the FBI can open an investigation whenever it becomes aware of a significant leak.

Administrative punishment is possible because the FBI is authorized to share the information from its investigations with the agency of the suspected leaker.

The numbers supplied to the Senate panel covered 2005 through 2009 -- a period that came after the opening of the investigation into who in the Bush administration disclosed the CIA identity of Valerie Plame, the wife of Iraq war critic Joe Wilson. In 2007, a jury convicted I. Lewis "Scooter" Libby, Vice President Dick Cheney's former chief of staff, of perjury, obstruction and lying to the FBI about Libby's conversations with reporters regarding Plame. President George W. Bush commuted Libby's 30-month prison term.

The time span covered by Mueller's answers to Capitol Hill ended before the current prosecution of former National Security Agency official Thomas Drake for allegedly being the source for news articles about the agency, including some that contained classified information. Drake has pleaded not guilty.

Mueller's responses to the Senate outlined some of the problems in tracking down leakers.

Classified information often can be properly disseminated to a large number of people in government, making identification of the leaker very unlikely, the Justice Department said.

Also, Justice Department policy requires that leak investigations focus on potential leakers rather than reporters, the recipients of leaks.

Balancing First Amendment freedoms with national security interests necessarily limits a prosecutor's access to the reporter who received classified information, the Justice Department told Whitehouse and the judiciary committee.

The Plame probe saw a number of reporters called to the witness stand at Libby's trial. Former New York Times reporter Judith Miller spent 85 days in jail rather than testify to a federal grand jury in the probe. She subsequently did so, and testified as well in open court at Libby's criminal trial.