Updated Forensics experts investigating the security breach on Sony's PlayStation Network found a file on one of the hacked systems that was titled “Anonymous” and contained the phrase “We are Legion,” the company's chairman told members of congress.

The revelation, made in a letter, (PDF here) that Sony Chairman Kazuo Hirai sent on Tuesday to members of the US House of Representatives, was used to support the company's contention that the massive security breach was carried out by members of Anonymous, the loosely organized griefer and hacker collective that sometimes uses the tag line: “We are Legion.”

“Just weeks before, several Sony companies had been the target of a large-scale, coordinated denial of service attack by the group called Anonymous,” Hirai wrote. “The attacks were coordinated against Sony as a protest against Sony for exercising its rights in a civil action in the United States District Court in San Francisco against a hacker.”

The sophistication of the PSN attackers, combined with the continuing DDoS attacks, made it hard for Sony admins to detect the compromise, which has resulted in the wholesale theft of personally identifiable information associated with 77 million accounts. Almost two weeks into the investigation of the hack, Sony learned that Station.com, its PC games site, was also breached, resulting in theft of PII associated with an additional 25 million accounts.

“Whether those who participated in the denial of services attacks were conspirators or whether they were simply duped into providing cover for a very clever thief, we may never know,” Hirai wrote. “In any case, those who participated in the denial of service attacks should understand that – whether they knew it or not – they were aiding in a well planned, well executed, large-scale theft that left not only Sony a victim, but also Sony's many customers around the world.”

Hirai's eight-page letter didn't leave open the very real possibility that the DDoS attacks were unrelated to the security breach. It wouldn't be a stretch for those who penetrated Sony's servers and stole the mountains of data to have left the file behind as a decoy intended to distract investigators from the true culprits.

The Chairman's letter came in response to questions members of the House Subcommittee on Commerce, Manufacturing, and Trade posed late last month. It provides a timeline and new details behind one of the largest data thefts ever.

The first sign of trouble surfaced on the afternoon of April 19, when US-based members of Sony's team discovered that some systems were rebooting even though they weren't scheduled to do so. They promptly launched a probe by reviewing server logs and about 24 hours later found the first signs indicating an “unauthorized intrusion had occurred and that data of some kind had been transferred off the PlayStation Network servers without authorization.”

Sony responded by taking the PSN completely offline and calling in a “recognized security and forensic consulting firm”. Investigators began the lengthy task of mirroring the PSN systems, which Hirai said consist of some 130 servers and 50 software programs.

On April 21, as the scope and complexity of the investigation grew, Sony brought in a second security firm. They didn't finish the mirroring the nine or 10 servers suspected of being compromised until April 22, and it took another day for investigators to “confirm that intruders had used very sophisticated and aggressive techniques to obtain unauthorized access, hide their presence from system administrators, and escalate privileges inside the servers,” Hirai said.

On Easter Sunday, Sony brought in a third forensics firm and by the following day investigators were for the first time able to confirm that PSN users names, birthdates, addresses, email addresses, and passwords were plundered. They found no evidence that credit card data was stolen, but they couldn't rule out the possibility.

Next page: PII from all PSN users lifted