Tweet Attacks | Anonymous Attacks DOJ Site After Feds Shut Down Megaupload

In one of the U.S. government's largest anti-piracy crackdowns ever, federal agents on Thursday arrested the leaders of and shut down Megaupload.com, a popular hub for illegal media downloads.

Hours later, Megaupload's fans turned the table on the feds. "Hacktivist" collective Anonymous said it set its sights on the U.S. Department of Justice and apparently knocked the agency's website offline.

"We are having website problems, but we're not sure what it's from," a DOJ spokeswoman told CNNMoney.

The DOJ website glitches came soon after various Twitter accounts associated with Anonymous took aim at the agency.

Anonymous's favorite weapon for these attacks is what's called a "distributed denial of service" (DDoS) attack, which directs a flood of traffic to a website and temporarily crashes it by overwhelming its servers. It doesn't actually involve any hacking or security breaches.

"One thing is certain: EXPECT US! #Megaupload" read one tweet from AnonOps that went out mid-afternoon.

One hour later, the same account tweeted a victory message: "Tango down! #Megaupload"

It was the largest attack ever by Anonymous, according to an Anonymous representative, with 5,635 people using a networking tool called a "low orbit ion cannon." A LOIC is software tool that aims a massive flood of traffic at a targeted site.

Universal Music's website also went down Thursday afternoon. The music company had been locked in a legal battle with Megaupload over a YouTube video that featured many of Universal Music's signed artists promoting Megaupload's site.

The websites of the Recording Industry Association of America and Motion Picture Association of America went down Thursday afternoon as well. On Twitter, AnonOps -- one of the main communications channels for the leaderless Anonymous collective -- took credit for the crashes.

An RIAA spokesman confirmed that the organization's website was intermittently offline. But he cast the attack as a minor hiccup.

"The fact that a couple of sites might have been taken down is really ancillary to the significant news today that the Justice Department brought down one of the world's most notorious file sharing hubs," he said.

By Friday morning, all but one of the subjects of Anonymous' attack were back online -- including the FBI's website, Warner Music Group and the U.S.