Thirteen members of a hacking collective that calls itself Anonymous were indicted in the US yesterday 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 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 at US district court in Alexandria, Virginia, the hackers’ tool of choice was a simple open-source application known as Low Orbit Ion Cannon, which requires very little technical know-how. Hackers simply posted a web link online that allowed volunteers to download an application that turned their computer into a “botnet,” or network of computers, that flooded targets like Visa.com and MasterCard.com with traffic until they crashed.

For instance, on September 16th 2010, a member of Anonymous posted on a bulletin board advertising an attack against the website for the Motion Picture Association of America, the indictment said. The instructions were to carry out the “first wave” of attack at 9pm. According to the indictment, the posting said: “This will be a calm, coordinated display of blood. We will not be merciful.”

Anonymous, a loose collective, has in the past carried out politically motivated attacks. In December, the group hacked into the Facebook and Twitter accounts belonging to the Westboro Baptist Church, after the church’s members had threatened to picket at a vigil for victims of the Newtown, Conn., elementary school shootings, claiming “God sent the shooter” in retaliation for Connecticut having legalized same-sex marriage.

In November, Anonymous began attacks against Israeli sites in retaliation for Israeli military strikes on Hamas. In February 2012, hackers claiming to belong to Anonymous took down the website for the Mexican National Chamber of Mines and claimed to have stolen its email. On a bulletin board, the hackers said the attack was in retaliation for Mexican miners’ “extreme” working conditions.

New York Times