A technical glitch gave the FBI access to the e-mail messages from an entire computer network - perhaps hundreds of accounts or more - instead of simply the lone e-mail address that was approved by a secret intelligence court as part of a national security investigation, according to an internal report of the 2006 episode.
FBI officials blamed an "apparent miscommunication" with the unnamed Internet provider, which mistakenly turned over all the e-mail from a small e-mail domain for which it served as host. The records were ultimately destroyed, officials said.
Bureau officials noticed a "surge" in the e-mail activity they were monitoring and realized that the provider had mistakenly set its filtering equipment to trap far more data than a judge had actually authorized.
The episode is an unusual example of what has become a regular if little-noticed occurrence, as American officials have expanded their technological tools: Government officials, or the private companies they rely on for surveillance operations, sometimes foul up their instructions about what they can and cannot collect.
The problem has received no discussion as part of the fierce debate in Congress about whether to expand the government's wiretapping authorities and give legal immunity to private telecommunications companies that have helped in those operations.
A report in 2006 by the Justice Department inspector general found more than 100 violations of federal wiretap law in the two previous years by the FBI, many of them considered technical and inadvertent.
Bureau officials said they did not have updated public figures but were preparing them as part of a wider-ranging review.
In the warrantless wiretapping program approved by President Bush after the Sept. 11, 2001, terrorist attacks, technical errors led officials at the NSA on some occasions to monitor communications entirely within the United States - in apparent violation of the program's protocols - because communications problems made it difficult to tell initially whether the targets were in the country or not.
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.
The episode was disclosed as part of a new batch of internal documents that the FBI 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 FBI had in place to guard against civil liberties abuses.
"How do we know what the FBI does with all these documents when a problem like this comes up?" Hofmann asked.
Michael Kortan, an FBI spokesman, said 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.
Kortan would not disclose the name of the Internet provider or the network domain because the national security investigation, which is classified, is continuing. The improperly collected e-mail was first segregated from the court-authorized data and later was destroyed through unspecified means. The individuals whose e-mail was collected apparently were never informed of the problem.
Kortan said he could not say how much e-mail was mistakenly collected as a result of the error, but he said the volume "was enough to get our attention."
Peter Eckersley, a staff technologist for the Electronic Frontier Foundation who reviewed the documents, said it would most likely have taken hundreds or perhaps thousands of extra messages to produce the type of "surge" described in the FBI's internal reports.
Kortan said that once the problem was detected the foreign intelligence court was notified, along with the Intelligence Oversight Board, which receives reports of possible wiretapping violations.
President Bush said Saturday that lawmakers' failure to renew an eavesdropping law will make it more difficult to track terrorists and "we may lose a vital lead that could prevent an attack on America."
Democrats faulted the president for "whipping up false fears and creating artificial confrontation."
At issue is a law that made it easier for the government to spy on foreign phone calls and e-mails that pass through the United States. The expiration time was midnight Saturday.
The president wanted the House to approve a Senate bill that would have renewed the law. Bush opposed a temporary extension; lawmakers left for a 12-day recess without extending the law.
The Senate measure included legal protections for telecommunications companies that helped the government wiretap U.S. computer and phone lines after the Sept. 11, 2001, attacks without clearance from a secret court that oversees such activities.