Police agencies worldwide are turning up the heat on a loosely organized group of WikiLeaks activists. On Thursday U.K. police arrested five people, and U.S. authorities said they'd executed more than 40 search warrants in the U.S. in connection with last month's Web-based attacks against companies that had severed ties with WikiLeaks.

Investigations are also ongoing in the Netherlands, Germany and France, the U.S. Federal Bureau of Investigation said Thursday.

Acting on information from German authorities, the FBI raided Dallas ISP Tailor Made Services last month, looking for evidence relating to one of the chat servers used by Anonymous. Another server was traced to Fremont, California's Hurricane Electric.

The actions come after Anonymous knocked websites for MasterCard, Visa and others offline briefly by recruiting volunteers to target them with a network stress-testing tool called LOIC (Low Orbit Ion Cannon). LOIC flooded the sites with data, making them unable to serve legitimate visitors. This type of attack is called a distributed denial of service (DDoS) attack.

Anonymous has also targeted PayPal, Amazon and the websites of Sarah Palin and the Swedish Prosecutor's Office with these attacks.

"[F]acilitating or conducting a DDoS attack is illegal, punishable by up to 10 years in prison, as well as exposing participants to significant civil liability," the FBI said in a press release.

Anonymous members say they want to send a message to companies that dropping WikiLeaks over its decision to publish classified documents is an attack on free speech.

These types of political DDoS attacks have become commonplace. Pro-Russia computer users used them to shut down much of Estonia's Internet infrastructure in 2007, and two years later, supporters of Iran's pro-democracy movement attacked a number of state-sponsored websites.

Anonymous has launched similar DDoS attacks in the past, too, knocking the websites of the Recording Industry Association of America and Scientology offline in recent years.

On Thursday, a Web page used to coordinate this latest round of DDoS attacks was offline, and the group's Twitter and Blogspot pages were silent.

The U.K.'s Metropolitan Police arrested five men aged 15 to 26 on Thursday. No arrests have been announced in the U.S. Last month, Dutch authorities arrested two teenagers in connection with the attacks.

Robert McMillan covers computer security and general technology breaking news for The IDG News Service. Follow Robert on Twitter at @bobmcmillan. Robert's e-mail address is robert_mcmillan@idg.com