U.S. military hackers have penetrated Russia’s electric grid, telecommunications networks and the Kremlin’s command systems, making them vulnerable to attack by secret American cyber weapons should the U.S. deem it necessary, according to a senior intelligence official and top-secret documents reviewed by NBC News.

American officials have long said publicly that Russia, China and other nations have probed and left hidden malware on parts of U.S critical infrastructure, “preparing the battlefield,” in military parlance, for cyber attacks that could turn out the lights or turn off the internet across major cities.

It’s been widely assumed that the U.S. has done the same thing to its adversaries. The documents reviewed by NBC News — along with remarks by a senior U.S. intelligence official — confirm that, in the case of Russia.