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Dutch authorities said Thursday they arrested a 16-year-old boy suspected of being involved in attacks on the websites of MasterCard and Visa that were allegedly mounted by sympathizers of document-leaking website WikiLeaks.

The boy, whom authorities didn't name but confirmed is Dutch, remains in custody and is due to appear before a judge on Friday. A lawyer for the boy couldn't be reached.

Dutch police believe he is part of a larger group of WikiLeaks sympathizers behind the so-called denial-of-service attacks, in which computers flood a server to prevent it from displaying a Web page, Mr. de Bruin said. Dutch authorities say the group conducting the attacks are doing so under an effort called Operation Payback. The police are continuing their investigation, including exploring the potential involvement of others in the Netherlands.

MasterCard Inc. and Visa Europe, which both recently suspended payments to WikiLeaks, are among a growing list of organizations and individuals that have suffered online attacks in recent days, in what appears to be an effort by hackers bent on exacting revenge for the document-leaking website.

The computer attacks have increased in intensity in recent days following the arrest Tuesday in London of WikiLeaks founder Julian Assange in connection with sexual-misconduct allegations in Sweden. Mr. Assange was denied bail and remains in custody pending a hearing in the U.K. next week. He has denied the allegations and hasn't been formally charged.

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The group behind the attacks appears to be a loose-knit collection of people identifying themselves as "Anonymous." It isn't clear who is in charge of or speaks for that group.

Click here for the full story from the Wall Street Journal.