A break-in targeting State Department computers worldwide last summer occurred after a department employee in Asia opened a mysterious e-mail that quietly allowed hackers inside the U.S. government's network.

In the first public account revealing details about the intrusion and the government's hurried behind-the-scenes response, a senior State Department official described an elaborate ploy by sophisticated international hackers. They used a secret break-in technique that exploited a design flaw in Microsoft software.

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