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A former senior intelligence official tells Fox News the justice department pulled its website offline Thursday evening, following signs hackers planned a denial of service attack on the site -- an assault clearly linked to arrests in New Zealand of a massive, worldwide piracy ring.

Hackers aligned with the global cyber-collective known as Anonymous claimed responsibility Thursday for taking down at least six prominent websites, including those of the U.S. Department of Justice and Universal Music Group. The hackers said their actions were retaliation for the shutdown of content-sharing site Megaupload.com and the arrest of several related to the site.

But the intelligence official tells a slightly different story, saying there were signs early on a cyberassault was imminent. The denial of service attack on the justice department website brought a surge of Internet traffic -- raising it from 50 hits a minute to beyond 1,000 hits -- at which point the DOJ took the site offline to install filters based on the incoming IP addresses.

Fox is told the incident lasted about 5 hours.

“The investigation is like a fresh crime scene,” the source told Fox News. “You got a body and a guy holding a smoking gun, but you don’t know if he’s the one who pulled the trigger. In this case whether Anonymous is really behind it.”

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On at least four other occasions, the source said, there were inklings the DOJ site was being targeted -- incidents that did not pan out. And this type of attack will only become more frequent in the future, General Dale Meyerrose with Harris Corp. told Fox News earlier this week.

"I think that we're going to start seeing more and more of these things as the tool of choice. Because it's high-value payback, but it's really low risk," he said. Meyerrose, who served as chief information officer in three major U.S. Air Force Commands and was the director of Command Control Systems, Headquarters North American Aerospace Defense Command, said a major arrest could serve to deter the brazen hackers.

"The [fear] that someone is actually going to suffer the consequence for carrying out either a propaganda, a retribution or maybe even a trial run of some future operation is, is really the key, the element of this whole thing."

The FBI’s special cyber taskforce was a central part of the investigation, and the Department of Homeland Security's Computer Emergency Readiness Team (US-CERT) stated that it assisted the DOJ and FBI in their investigation.

The cyberattack was one in a series of assaults Thursday night, all of which the Anonymous collective claimed responsibility for. Following outages at Universalmusic.com, the websites of the Recording Industry Association of America (RIAA) and the Motion Picture Association of America (MPAA), the group threatened that the FBI's website would be hacked next.

"Word has it that FBI.gov is next to go down ... will report when it is," a Twitter account affiliated with the group said.

The account, under the name @YourAnonNews identifies itself on Twitter as the infamous hacker collective, saying "We are Anonymous, We are legion, We never forgive, We never forget, Expect us."

Earlier Thursday, the FBI shut down Megaupload.com and charged the site's founders and five others with Internet piracy crimes for running "an international organized criminal enterprise."

"The government takes down Megaupload? 15 minutes later Anonymous takes down government & record label sites," the group wrote earlier Thursday.

"Megaupload was taken down w/out SOPA being law. Now imagine what will happen if it passes. The Internet as we know it will end. FIGHT BACK," the group added, in a reference to the ongoing battle in Congress over the controversial Stop Online Piracy Act (SOPA).

The proposed legislation would crack down on the online sharing of pirated copies of music, movies and other material.

Investigators said there was no connection between arrests in their two-year investigation of Megaupload.com and the political firestorm that erupted this week over the pending vote on SOPA.

Reporting by Catherine Herridge. Newscore contributed to this report.