The FBI may have committed as many as 6,400 intelligence violations in the course of its use of national security letters, Justice Department Inspector General Glenn Fine told lawmakers Tuesday.
National security letters allow FBI investigators to obtain personal records such as Internet, phone and financial information without going first to a judge for a warrant.
Fine explained the method his office used to estimate the number of violations.
"The FBI's 10 percent review of field office NSLs found at least 640 potential intelligence violations from 2003 through 2006," Fine told members of a House Judiciary subcommittee.
"Extrapolating the results of the FBI's 10 percent statistical sample to the full number of NSLs means that the total number of possible intelligence violations among all NSLs issued over the four-year period could be as high as 6,400," Fine continued.
Fine also described his office's oversight since it disclosed last year that the FBI had issued more than 140,000 national security letters from 2003 to 2005.
Last month, Fine released a follow-up report, which found that the FBI had issued 49,425 national security letters in 2006 alone.
The inspector general's reviews prompted the FBI and Justice Department's National Security Division to begin internal audits of how NSLs are used.
Under federal law, all intelligence agencies are required to self-report possible violations of the law to the president's Intelligence Oversight Board.
At the hearing, FBI general counsel Valerie Caproni said that many of the violations "involved third-party errors or inattention to detail." According to FBI officials that type of infraction includes clerical errors.
Asked about the 6,400 figure after the hearing, Caproni disputed it and said, "Ninety percent of those are third-party errors."
A Freedom of Information Act lawsuit by the Electronic Frontier Foundation against the FBI previously disclosed documents that show the FBI overcollected data from entire e-mail servers instead of only the intended suspects' communications.
"There was no violation of a customer's Fourth Amendment rights," Caproni said when asked about the overcollection of data in similar cases.
A document released Monday as part of the lawsuit disclosed that the FBI sought to use a national security letter to obtain information about an Egyptian student at North Carolina State University for potential links to suicide bombers in the deadly 2005 London attacks.
The documents showed that officials at FBI headquarters sought to obtain information from the NSL instead of using a traditional grand jury subpoena.
In an e-mail dated July 21, 2005, an FBI official whose name was redacted in the documents noted, "The director would like to use this as an example tomorrow as to why we need administrative subpoenas to fight the war on terror. In particular he would like to know how much extra time was spent having to get the grand jury subpoena."
The request came at a time when the FBI was pushing for administrative subpoena powers to be used for terrorism investigations.
On July 27, 2005 Mueller told the Senate Judiciary Committee, "We have relied on national security letters and FISA orders for business records. And although both are useful and important tools in our national security investigations, administrative subpoena power would greatly enhance our abilities to obtain information."