The malware, one congressional official said, was essentially “a ticking time bomb” that could give China the power to interrupt or slow American military deployments or resupply operations by cutting off power, water and communications to U.S. military bases. But its impact could be far broader, because that same infrastructure often supplies the houses and businesses of ordinary Americans, according to U.S. officials.

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.