A hacker group behind online attacks on companies that withdrew services to WikiLeaks busted through the defenses of a computer security firm working with federal agents to expose their identities.

Hackers operating under the banner "Anonymous" took credit for breaking into the website of HBGary Federal, stealing tens of thousands of email messages and temporarily routing traffic to a page with a vitriolic message.

"You've tried to bite the Anonymous hand," a copy of the message online Monday stated. "You angered the hive and now you are being stung."

Efforts to visit HBGary's website on Monday were met with an automated post saying the page was "under construction."

Plundered email accounts included that of HBGary chief executive Aaron Barr, whose separate Twitter account was also reportedly compromised by someone who "tweeted" personal information about him and rude messages.

Stolen email messages were made available online at a popular peer-to-peer file sharing website, Chester Wisniewski of Sophos computer security firm said in an online post regarding the hack.

The HBGary hack was more sophisticated than the distributed denial of service (DDoS) attacks last year on the Amazon, Visa and MasterCard websites in apparent retaliation for their decisions to stop working with WikiLeaks.

WikiLeaks had triggered political ire in Washington for its publication of thousands of classified US diplomatic cables and military reports from Iraq and Afghanistan.

"Unlike the DDoS attacks for which Anonymous has made headlines in recent months, this incident involved true hacking skills," Wisniewski said.

In a typical DDoS attack, a large number of computers are commanded to simultaneously visit a website, overwhelming its servers, slowing service or knocking it offline completely.

HBGary had been working to expose the culprits behind the DDoS attacks and was poised to sell identifying information about members of Anonymous to the FBI, according to Wisniewski.

Last month, British police arrested five people and the US Federal Bureau of Investigation launched raids across the United Staes as part of a probe into cyberattacks by Anonymous.