The arrests targeted individuals who participated in a distributed denial of service (DDoS) attacks on the Web sites of Paypal, the online payment Web site in December, 2010, according to a 15 count indictment published in U.S. District Court in San Jose California. Security experts are asking that very question, as Anonymous promises retribution for the arrests and more hacks. The arrests and searches appear similar to those conducted in Europe, including the arrest of a Dutch teenager for participating in DDoS attacks on Mastercard and Visa.
Source: https://threatpost.com/fbi-arrests-searches-do-little-quiet-anonymous-072111/75461/