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For the last two months, senior government officials and private-sector experts have paraded before Congress and described in alarming terms a silent threat: cyberattacks carried out by foreign governments. Robert S. Mueller III, the director of the F.B.I., said cyberattacks would soon replace terrorism as the agency's No. 1 concern as foreign hackers, particularly from China, penetrate American firms’ computers and steal huge amounts of valuable data and intellectual property.