The first public hints of the malware campaign began to emerge in late May, when Microsoft said it had detected mysterious computer code in telecommunications systems in Guam, the Pacific island with a vast American air base, and elsewhere in the United States. But that turned out to be only the narrow slice of the problem that Microsoft could see through its networks.

More than a dozen U.S. officials and industry experts said in interviews over the past two months that the Chinese effort goes far beyond telecommunications systems and predated the May report by at least a year. They said the U.S. government’s effort to hunt down the code, and eradicate it, has been underway for some time. Most spoke on the condition of anonymity to discuss confidential and in some cases classified assessments.