FBI personnel who used misleading emergency letters to acquire thousands of Americans' phone records are the subject of a criminal investigation, top bureau officials told civil liberties groups Monday.

The unprecedented criminal probe, revealed at an outreach meeting led by FBI director Robert Mueller and general counsel Valerie Caproni at FBI headquarters, is looking at the actions of an antiterrorism team known as the Communications Analysis Unit, according to two people who attended the meeting independently and who informed Wired News, requesting anonymity.

The privately disclosed investigation would mark the first time government officials have faced possible prosecution for misuse of Patriot Act investigative tools, and highlights the seriousness of recent reports about the FBI's misuse of a powerful self-issued subpoena known as a National Security Letter.

Though warned in 2001 to use this power sparingly, FBI agents issued more than 47,000 National Security Letters in 2005, more than half of which targeted Americans.

However, the Justice Department's Inspector General reported (.pdf) in March that the office issued 739 "exigent letters" to AT&T, Verizon and MCI seeking information on more than 3,000 phone numbers.