The FBI confirmed the hacking group listened in on a conference call last month between the bureau, Scotland Yard and other foreign security agencies about their joint investigation of Anonymous, The New York Times reported Friday.
A 16-minute recording of the conference call was posted to the Internet by the international hacking group.
"The FBI might be curious how we're able to continuously read their internal comms for some time now," the group boasted on Twitter.
Anonymous had not in fact hacked into the conference call or any other FBI facilities, an official said, but rather had obtained an e-mail giving the time, telephone number and access code for the call.
The e-mail was sent Jan. 13 to people at the FBI, Scotland Yard, and agencies in France, Germany, Ireland, the Netherlands and Sweden.
One of the recipients, a foreign police official, apparently forwarded the e-mail to a private account where it was intercepted by Anonymous.
"It's not really that sophisticated," an FBI official, speaking on condition of anonymity, said.
No FBI system was compromised but security was more difficult when agencies in multiple countries were involved, he said.
"We're always looking at ways to make our communications more secure, and obviously we'll be taking a look at what happened here," he said.