Kazuo Hirai, chairman of Sony Computer Entertainment, said in a letter Wednesday to a Congressional subcommittee investigating the incident that the company's security team turned up evidence on Sunday pointing to Anonymous as the culprits.
The team was probing a separate compromise, on Sony Online Entertainment's network, when they discovered that the hackers -- who exposed more than 100 million accounts in total -- planted a file named "Anonymous" that contained the words "We are Legion," part of the group's motto.
But in an interview Wednesday afternoon with SCMagazineUS.com, Barrett Brown, the sometimes-spokesman for Anonymous, denied the group's involvement.
"They could've just as easily left documents saying, 'Congress. We investigate steroid use in baseball,'" joked Brown. "Anonymous has no record in engaging in credit card theft and resell, and if we did, the FBI would've already come down on us."
Instead, he said the blame likely rests with a professional criminal outfit, possibly one based in Eastern Europe, who used Anonymous' ongoing feud with Sony over piracy allegations to shift suspicion away from it.
"Any clever thief of that sort would be inclined to leave a document laying blame to someone else," he said. "We're all mystified by this. Everyone just assumes -- knows -- it's some criminal group. But it wasn't us."
In his letter to the House Subcommittee on Commerce, Manufacturing and Trade, Hirai referenced distributed denial-of-service (DDoS) attacks launched in early April by Anonymous against Sony.
Anonymous has admitted that it unleashed the attacks as revenge for Sony pursuing lawsuits against two hackers who allegedly violated federal copyright law by jailbreaking the PlayStation 3. But Brown said the true PSN hackers likely are leveraging this public dispute as cover to steal credit card information.
There also exists the possibility that the culprits are members of the federal contractor or intelligence communities, who perpetrated the intrusion as part of a "false flag operation" to smear Anonymous, Brown said.
"We're under this massive assault in various directions by people who would love to discredit us," he said.
The group clearly has shifted its mission away from pranks and toward more "hacktivist" endeavors in recent years, including launching attacks against the anti-gay Westboro Baptist Church; the Church of Scientology; and MasterCard and Amazon, after they ceased doing business with whistleblower website WikiLeaks.
Arguably Anonymous' most notable operation came in February, when members hacked government cybersecurity services firm HBGary Federal to expose shady practices.