The Anonymous hacking group recruited unwitting accomplices in Thursday's attacks against U.S. government sites, a security researcher said today.

The distributed denial-of-service (DDoS) attacks began Thursday just hours after the U.S. Department of Justice announced arrests of four men associated with the popular Megaupload "cyberlocker" site on charges of copyright infringement, money laundering and racketeering.

Federal authorities shuttered Megaupload.com and other sites, and seized assets belonging to the company, including hundreds of servers. Three of the seven men indicted remain at large, but four were arrested in New Zealand by local authorities and face extradition to the U.S.

Almost immediately, Anonymous retaliated with DDoS attacks against Justice's website, and those operated by Universal Music, the Recording Industry Association of America (RIAA), the Motion Picture Association of America (MPAA), and others. Some of those sites were inaccessible during parts of Thursday.

In a message on Twitter and in a blog post, Anonymous claimed Thursday's DDoS attacks were its largest ever, and said that 5,600 people collaborated in the assaults.

Previously, Anonymous had said that its followers were using the Low Orbit Ion Cannon (LOIC) tool, a favorite of the group since its first widespread DDoS attacks in December 2010.

But some of the 5,600 who participated may have done so unwittingly, said Graham Cluley, a senior technology consultant with U.K.-based antivirus vendor Sophos.

According to Cluley, members of Anonymous distributed links via Twitter and elsewhere that when clicked automatically launched a Web version of LOIC. The links pointed to a page on PasteHTML.com, a free HTML code-hosting site, which in turn executed some JavaScript to fire LOIC at Anonymous-designated targets.

Many of those messages said nothing about LOIC or that clicking the link shanghaied the user into the DDoS attack, Cluley said, noting several Twitter messages as examples.

In an email reply to questions today, Cluley said that while the links were launching LOIC against more than one website, "It's clear that justice.gov is getting a lot of attention."

The Department of Justice's website was operating normally early Friday.

Anonymous is still recruiting people to its campaign. A quick search of Twitter using a string published on Gawker.com indicated that the link was being shared Friday morning at the rate of about 10 to 18 times per minute on the micro-blogging site.

On a Sophos blog, Cluley reminded readers that DDoS attacks were illegal, and cautioned users to be wary of clicking links.

"Anonymous might be hoping that participants could argue that they did not knowingly assist in the DDoS attack, and clicked on the link in innocence without realizing what it would do," said Cluley.

Gregg Keizer covers Microsoft, security issues, Apple, Web browsers and general technology breaking news for Computerworld. Follow Gregg on Twitter at @gkeizer, on Google+ or subscribe to Gregg's RSS feed . His e-mail address is gkeizer@computerworld.com.

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