China and "probably one or two" other countries could shut down critical computer networks that could force U.S. power and water grids, aviation systems, and financial services offline.
National Security Agency director Adm. Mike Rogers said on Thursday in testimony to the U.S. House of Representatives' Intelligence Committee that hackers have performed "reconnaissance" missions in order to work out how networks function.
Those missions can be used to determine weak spots, Rogers hinted.
"What concerns us is that access, that capability can be used by nation states, groups, or individuals to take down that capability," he said, according to a Reuters transcript.
He named China as one of the countries with that capability, but declined to expand to others, saying only that the information is classified.
Rogers, whoin the months following the Edward Snowden leaks first came to light, said such an attack was "a matter of when, not if."
The testimony comes in the same week the counter-surveillance Freedom Act was shot down in the U.S. Senate, partly in thanks to Republicans voting against the bill. However, some -- notably Sen. Rand Paul (R-KY) --because the bill contained an extension of the controversial Patriot Act, which was signed into law a month after the September 11 attacks in 2001.
The Patriot Act drove a number of the NSA's powers in collecting billions of phone records, a program dubbed the "215 Program" for its namesake space in the law books, which remained secret until Snowden revealed the program as his debut leak.
The U.S. agency was also found to have, in subsequent reports, snooped on world leaders, and forced technology companies to hand over data it stored on its customers -- all in the name of national security.