The Associated Press reports that "About a dozen times in the last decade, sophisticated foreign hackers have gained enough remote access to control the operations networks that keep the lights on, according to top experts who spoke only on condition of anonymity due to the sensitive nature of the subject matter."
"The public almost never learns the details about these types of attacks -- they're rarer but also more intricate and potentially dangerous than data theft. Information about the government's response to these hacks is often protected and sometimes classified; many are never even reported to the government."
"These intrusions have not caused the kind of cascading blackouts that are feared by the intelligence community. But so many attackers have stowed away in the largely investor-owned systems that run the U.S. electric grid that experts say they likely have the capability to strike at will."