The American FBI recently revealed that it had found 14 suspected leakers (of classified information) between 2005-9. That's only 7.5 percent of the 186 requests to find suspected leakers. However, the FBI only agreed to investigate 14 percent of those 186 requests, and succeeded in a little over half of those investigations. Moreover, none of the leakers the FBI identified were prosecuted. What is going on here?
These leaks, mostly in the Washington, DC, area, are quite common, with about a hundred a year since the 1990s. Most (60 percent) are not even investigated. And since 1995, only two of these 600 investigations have led to a prosecution. One was a retired NSA engineer, while the other Lewis Libby, who was convicted not of leaking the identity of a CIA employee (another government employee, who was not prosecuted, was responsible), but of lying to the FBI during the investigation.
Many leaks are not investigated because they were deliberate, sometimes even authorized by a senior political official. Many of those investigated go nowhere because you need strong evidence to nail the leaker. NSA engineer Thomas Drake was caught using encrypted email to do his leaking. The NSA is very good at deciphering encrypted material, but they won't talk about what they might have done to break this case. Another problem is that if the leaker has powerful allies within the government, the investigation can easily be derailed, or stonewalled. FBI agents investigating leaks know to expect this, and back off as needed.