Thirteen members of a hacking collective that calls itself Anonymous were indicted Thursday on charges that they conspired to coordinate attacks against prominent websites.

The 13 are accused of bringing down at least six websites, including those belonging to the Recording Industry Association of America, Visa and MasterCard.

The attacks caused “significant damage to the victims,” the indictment said.

The attacks, carried out from September 2010 to January 2011, were part of a campaign called Operation Payback, which started as an effort to support file-sharing sites but later rallied around WikiLeaks and its founder, Julian Assange.

Hackers took down the sites by inflicting a denial of service, or DDoS, attack, in which they fired Web traffic at a site until it collapsed under the load. Though the indictment mentions 13 hackers, thousands more participated in the attack by clicking on Web links that temporarily turned their computers into a digital fire hose aimed at each victim, in this case the websites.

According to the indictment, which was handed up in U.S. District Court in Alexandria, Va., the hackers’ tool of choice was a simple open-source application known as Low Orbit Ion Cannon, which requires very little technical know-how.