Forensics teams have tracked the attack that netted the information to a file named "Anonymous" on an internal Sony including the words "We are Legion", which is part of the hacker group's motto.

"Just weeks before, of a large-scale, coordinated denial of service attack by the group called Anonymous," says Kazuo Hirai, Sony's chairman, in the letter.

PAST DDOS DEBACLES:

That attack and the threat of more had Sony network staff focused on DDoS defense, which might have distracted them from discovering the breach earlier, the letter says, "perhaps by design."

"Whether those who participated in the denial of service attacks were conspirators or whether they were simply duped in to providing cover for a very clever thief, we may never know. 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."