Chinese cyberspies, who targeted the personal Gmail accounts of top U.S. officials, are trying to gain access to computers belonging to China specialists and defense contractors who circulate in and out of government and talk regularly with those in power, according to security experts who have tracked these schemes.

The stealth infiltration campaign, similar in tactics to the Gmail scheme that Google Inc. disclosed last week, represents cyberspies' efforts to circumvent the high security walls on official government email...