Ning Wen and his wife were arrested last fall at their home office in Manitowoc, Wis., for allegedly sending their native China $500,000 worth of computer parts that could enhance missile systems. As these naturalized citizens await trial, similar episodes in Mount Pleasant, N.J., and Palo Alto, Calif., point only to the tip of the iceberg, according to FBI officials keeping tabs on more than 3,000 companies in the U.S. suspected of collecting information for China. A hotbed of activity is Silicon Valley, where the number of Chinese espionage cases handled by the bureau increases 20% to 30% annually. Says...