The Internet vigilante group Anonymous denied responsibility for a cyber-attack on Sony Corp's networks that exposed the personal data of more than 100 million video gamers.

Let's be clear, we are legion, but it wasn't us. You are incompetent Sony, Anonymous said on its Blog (anonops.blogspot.com) on Thursday.

Anonymous is a grass-roots cyber-vigilante group that launched attacks in December that temporarily shut down the sites of MasterCard Inc and Visa Inc using simple software tools freely available over the Internet.

Sony spokesman Dan Race declined to comment on the latest Anonymous press release, citing an ongoing investigation.

The group's denial came a day after Sony charged Anonymous was indirectly responsible for the attack on the Japanese electronics giant's networks. Sony made the accusation in a letter to members of Congress conducting an inquiry into one of the largest computer breaches in history.

On Thursday, Anonymous began threatening a new target: Viacom Inc, the entertainment giant behind MTV's Jersey Shore and the hit film Rango.

The group attacked Viacom for pulling its content from YouTube and suing the web-video site in a high-profile copyright infringement case dating back to 2006.

Anonymous demands from Viacom a public press release to admit and apologize for the fraud and crimes that they have committed, the group said in a press release of its own.

A spokesperson for Viacom declined comment on the matter.

SONY ATTACK

Sony said its video game network was hacked at the same time it was defending itself against a major denial-of-service attack by Anonymous. A denial-of-service attack makes a server or system unavailable by overwhelming its network with Internet traffic.

The attack that stole the personal data of millions of Sony customers was launched separately, right when the company was distracted protecting itself against the denial-of-service campaign, Sony said.

The company said it was not sure whether the organizers of the two attacks were working together.

Barrett Lyon, a security expert who specializes in helping companies defend themselves against denial-of-service attacks, believes Anonymous is at least indirectly responsible for the Sony hacking.

At the very least they planted the seed that focused nefarious intention toward Sony, he said.

It could have been something that the Anonymous group started, but really couldn't stop. They work like a lynch mob and it just takes one crazy person to keep going.

(Reporting by Jim Finkle; additional reporting by Yinka Adegoke; editing by Richard Chang and Andre Grenon)

Cybercrime Sony U.S.