Past violations by the government have also included continuing a wiretap for days or weeks beyond what was authorized by a court, or seeking records beyond what were authorized. The 2006 case appears to be a particularly egregious example of what intelligence officials refer to as “overproduction”  in which a telecommunications provider gives the government more data than it was ordered to provide.

The problem of overproduction is particularly common, F.B.I. officials said. In testimony before Congress in March 2007 regarding abuses of national security letters, Valerie E. Caproni, the bureau’s general counsel, said that in one small sample, 10 out of 20 violations were a result of “third-party error,” in which a private company “provided the F.B.I. information we did not seek.”

The 2006 episode was disclosed as part of a new batch of internal documents that the F.B.I. turned over to the Electronic Frontier Foundation, a nonprofit group in San Francisco that advocates for greater digital privacy protections, as part of a Freedom of Information Act lawsuit the group has brought. The group provided the documents on the 2006 episode to The New York Times.

Marcia Hofmann, a lawyer for the privacy foundation, said the episode raised troubling questions about the technical and policy controls that the F.B.I. had in place to guard against civil liberties abuses.

“How do we know what the F.B.I. does with all these documents when a problem like this comes up?” Ms. Hofmann asked.

In the cyber era, the incident is the equivalent of law enforcement officials getting a subpoena to search a single apartment, but instead having the landlord give them the keys to every apartment in the building. In February 2006, an F.B.I. technical unit noticed “a surge in data being collected” as part of a national security investigation, according to an internal bureau report. An Internet provider was supposed to be providing access to the e-mail of a single target of that investigation, but the F.B.I. soon realized that the filtering controls used by the company “were improperly set and appeared to be collecting data on the entire e-mail domain” used by the individual, according to the report.

The bureau had first gotten authorization from the Foreign Intelligence Surveillance Court to monitor the e-mail of the individual target 10 months earlier, in April 2005, according to the internal F.B.I. document. But Michael Kortan, an F.B.I. spokesman, said in an interview that the problem with the unfiltered e-mail went on for just a few days before it was discovered and fixed. “It was unintentional on their part,” he said.