The group Anonymous announced the attack via Twitter, saying it is part of a campaign to expose government and corporate failure to secure computer networks.

The hackers said in a message posted on the Internet that they wiped out 4 gigabytes of Booz Allen source code and stole 90,000 email addresses, although they apparently were able to get only encrypted versions of the email passwords.

Booz Allen declined comment, other than to say, "We generally do not comment on specific threats or actions taken against our systems."

Jim Lewis, a cybersecurity expert with the Center for Strategic and International Affairs, was less than impressed with the break-in and said he didn't believe it would hurt them in the long run.

"I'm not sure it's a big deal," he said in email comment. "They say they got lots of email addresses? Sounds like a scavenger hunt more than a hack."

But Anup Ghosh, founder of the Invincea security company, said Booz Allen's clients could have their confidence shaken because of the security breach. "It will hurt Booz Allen. They will now lose potential customers," he said.

And the company's troubles may not be over. The email addresses will be valuable to potential hackers because the owners can be sent spoof emails designed to entice them to click on a link to download malicious software. "Usually five to 20 percent (of recipients) will click if it's a well-crafted email," said Ghosh.

The more serious hacks have been into companies like web search giant Google Inc or a major intrusion into Pentagon networks in 2009. Both were blamed on Chinese entities.

Another rogue group, Lulz Security, broke into U.S. Senate server, brought down the CIA website and struck an Arizona police website last month.

Anonymous became famous late last year for attacking the companies and institutions that oppose WikiLeaks and its founder, Julian Assange.

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