A screenshot of the Justice Department Web site, from 2009. (Justice Department)

Anonymous, a group of Internet hackers, claimed responsibility for the attack via Twitter, as it usually does.

Barrett Brown, the Dallas-based founder of an online think tank that works with Anonymous, confirmed in a phone interview that Anonymous took down the Web sites using “distributed denial of service” attacks — essentially bombarding the sites with an overwhelming amount of traffic.

Several attempts to load the site, www.Justice.gov, failed late Thursday afternoon.

In a statement, the Justice Department said late Thursday that the site was “experiencing a significant increase in activity, resulting in a degradation in service. The Department is working to ensure the website is available while we investigate the origins of this activity, which is being treated as a malicious act until we can fully identify the root cause of the disruption.”

A Twitter feed published by Anonymous members said the group had also attacked UniversalMusic.com and the Web sites of the Recording Industry Association of America and the Motion Picture Association of America.

Kim Schmitz and three other executives of Megaupload.com were arrested Thursday in New Zealand at the request of U.S. officials, the Justice Department said Thursday. Two other defendants are at large, officials said. (Read the indictment here.)

Before the company’s site was taken down, Megaupload posted a statement saying allegations that it facilitated massive breaches of copyright laws were “grotesquely overblown,” according to the Associated Press.

“The fact is that the vast majority of Mega’s Internet traffic is legitimate, and we are here to stay. If the content industry would like to take advantage of our popularity, we are happy to enter into a dialogue. We have some good ideas. Please get in touch,” the statement said.

Brown, who said he is a former journalist working on a book about Anonymous, said Anonymous hackers are devising a new attack against Democratic members of Congress who are still endorsing the SOPA legislation. The operation’s name: Donkey Punch. “We’re trying to decide if we’re going to target one Congress member first or warn them first,” Brown said. “Another method would be to go after their donors too.”

Brown added that Anonymous hackers might also figure out a way to ensure that certain Congress members' names would be linked to their support of SOPA.

“Operation Donkey Punch is definitely going to involve bringing attention to the Congress members through creative means,” Barrett said. “We have means to tie someone’s name to something forever using search engine optimization.”

Hackers have targeted Congress before: In Jan. 2010, hackers seized at least 49 Web sites operated by House lawmakers that in some cases were offline for more than a day.

Follow Ed O’Keefe on Twitter: @edatpost

Further reading:

FAQ: What’s next for SOPA and PIPA?

Megaupload shutdown: SOPA supporters versus Alicia Keyes, P. Diddy?

For more, visit PostPolitics and The Fed Page.

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