The hacker group Anonymous said it launched its largest-ever online attack in retaliation for the U.S. government’s shutdown of one of the world’s largest file-sharing websites, Megaupload, for alleged copyright infringements.

More than 5,000 of its members had collaborated to bring down the websites of the Justice Department, the Motion Picture Association of America and the Recording Industry Association of America, the hacker collective said on Thursday. All three sites were inaccessible on Thursday.

The group said in a message on its Twitter account @YourAnonNews that 5,635 people were using denial-of-service attacks to target the websites, including those of the Federal Bureau of Investigation and the White House, although both those sites appeared to be working.

The action came after federal prosecutors unsealed an indictment against seven people connected with Megaupload, which they said earned $175 million while costing copyright holders more than $500 million in lost revenue.

Megaupload founder Kim Dotcom, whose real name is Kim Schmitz, was among four of the accused arrested on Friday in New Zealand.