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.

Unit employees, who are not authorized to request records in investigations, sent form letters to telephone companies to acquire detailed billing information on specific phone numbers by falsely promising that subpoenas were already in the works.

According to a third source, FBI officials also said at the meeting that some bureau employees have already been granted immunity from prosecution in the investigation. The third source, who also spoke on condition of anonymity, did not recall, however, that FBI officials described the investigation as "criminal."

FBI spokesman Richard Kolko confirmed that the meeting took place but declined to comment on the content of the conversation, saying only, "The FBI does not confirm or deny investigations."

Neither the Justice Department's Office of the Inspector General nor the FBI's Office of Professional Responsibility returned calls for comment.

While the scope of the alleged investigation is unknown, investigators could be examining whether the unit violated the Electronic Communications Privacy Act, or committed fraud by falsely swearing that subpoenas were being prepared.

National Security Letters are self-issued subpoenas that allow investigators in terrorism and espionage cases to require phone companies, banks, credit reporting agencies and internet service providers to turn over records on Americans considered "relevant" to an investigation. Those records are then fed into three computer systems, including a shared data-mining tool known as the Investigative Data Warehouse.

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.

Justice Department officials, including Attorney General Alberto Gonzales, have downplayed the gravity of the reported errors while attempting to mollify critics by promising to strengthen internal oversight.

The Communications Analysis Unit, part of the FBI's Communications Exploitation Section based in the agency's headquarters building, is tasked with analyzing terrorist communications and providing intelligence to the FBI's Counterterrorism Division. However, because it only supports investigators, unit employees cannot issue subpoenas and instead need to have counterterrorism investigators do so.

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. The letters stated: "Due to exigent circumstances, it is requested that records for the attached list of telephone numbers be provided. Subpoenas requesting this information have been submitted to the U.S. Attorney's Office who will process and serve them formally to (Phone Company Name) as expeditiously as possible."

However, no such subpoenas had been filed with U.S. Attorneys and only later were some of the requests followed up with proper legal process, according to the Inspector General's report.

Several of the letters included requests for records for more than 100 phone numbers.

Bassem Youssef, the current head of the Communications Analysis Unit, told Congress in March that key FBI lawyers knew about the problem in 2005, when he notified them and put an end to the false letters.

Youssef first noticed the problem with the letters in 2005 when he took over the unit and quickly brought the matter to the attention of his supervisor and the FBI's Office of the General Counsel, according to a March letter (.pdf) sent by his lawyer, Stephen Kohn, to Sen. Chuck Grassley.

"At all times, the (National Security Law Branch) and the FBI (Office of the General Counsel) knew that the field offices and operational units were non-compliant in obtaining the legal documentation," Kohn wrote.

Youssef is currently suing the FBI for retaliating against him for complaining that the bureau was wasting his Arabic-language skills and antiterrorism experience. He attempted to get proper National Security Letters filed to provide post-facto legal backing for the exigent letters but was hampered by uncooperative field offices, according to the Senate letter.

Kohn did not respond to requests for comment.