The mourning for the victims of Monday's attack reached northeastern China, home of the third person killed in the blasts, Lu Lingzi.

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If China made itself immune from outside attack, it could still be vulnerable to botnets run from within the country, says Allan Paller, director of research at the SANS Institute.

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China has bought 12 Russian Kilo-class diesel attack submarines.

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The group were arrested in Laos and brought to China in May last year, after Beijing said the attack had happened on board Chinese-flagged ships.

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It describes its approach as "protect and attack" - protect its lead in markets like China, while attacking new territories and product areas.

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