The FBI confirmed Friday that it had been forced to shut down its Internet-facing unclassified network, but disputed a report that the incident had left the agency unable to e-mail counterparts in other intelligence and law enforcement agencies. "The external, unclassified network was shut down by the FBI as a precautionary measure," the FBI . "Within 48 hours of identifying the issue and mitigating risks, e-mail traffic was largely restored to the external, unclassified network."

FBI agents can send e-mail on the agency's more secure internal network or via BlackBerry, but many use this unclassified network to send messages via a Web-based e-mail system, said a source familiar with the situation. That webmail service was down throughout the week and continued to be unavailable for some users, the source said.

"We can e-mail to anyone ... and [we] have Internet access. We also have a secure e-mail system that connects all 400+ offices around the country and 60 offices overseas," FBI spokesman Paul Bresson said in an e-mail message.

The FBI did not provide details on the security incident, but it looks as though hackers may have used maliciously encoded file attachments to hack into the network. In its statement, the FBI said it was now blocking users from sending or receiving attachments on the unclassified network "to give our technicians time to scan all the attachments that came into the e-mail system to make sure we have identified and mitigated all threats to the network."