The system, Carnivore, was supposed to capture e-mail from the target of the investigation, but also saved messages from other individuals, according to an FBI memo made public on Tuesday by the Electronic Privacy Information Center (EPIC), which obtained the memo under the US Freedom of Information Act.
"The software was turned on and did not work correctly," the memo stated, and an "FBI technical person was apparently so upset that he destroyed all" saved e-mail messages, including the ones related to the investigation.
The FBI, in other now public documents written the following week, said that e-mail surveillance with Carnivore could lead to "improper capture of data" and that "unauthorised interceptions not only can violate a citizen's privacy, but also can seriously 'contaminate' ongoing investigations".
The Carnivore e-mail surveillance tool, known officially as DCS1000, works by capturing data packets that pass through the systems of an Internet service provider. To do this, a box with the Carnivore software is installed on the ISP's network.
Civil rights groups have criticised the system.
EPIC sees the FBI documents as evidence that Carnivore is "a powerful but clumsy tool that endangers the privacy" of citizens. In a statement, EPIC has demanded that use of the system to be suspended "until the questions surrounding it can finally be resolved".
Nobody at the FBI was immediately available for comment. However, an anonymous bureau official quoted in the online edition of The New York Times said that Carnivore, in this particular case, gathered too much information because it was not set up correctly, and that the e-mail messages that the memo claimed had been deleted were recovered.