Story highlights A U.S. official close to the investigation said the power outage was caused by a sophisticated attack using destructive malware

U.S. systems aren't any more protected than those breached in Ukraine, the U.S. official said

Washington (CNN) U.S. investigators have found evidence to confirm what is believed to be the first-of-its-kind cyberattack on a power grid that caused a blackout for hundreds of thousands of people in Ukraine in December.

A U.S. official close to the investigation said the power outage was caused by a sophisticated attack using destructive malware that wrecked computers and wiped out sensitive control systems for parts of the Ukrainian power grid.

For years, U.S. officials have expressed worry about the vulnerability of the U.S. power grid. And the U.S. investigation of the Ukrainian attack has confirmed what until now has been largely theoretical: that cyberwarfare can be used to disable the U.S. power grid. U.S. systems aren't any more protected than those breached in Ukraine, the U.S. official said.

Ukrainian authorities have blamed Russia for the outage, saying it was part of the Russian government's pattern of undeclared war against its neighbor. Almost immediately, investigators found indications of a malware called BlackEnergy.

The U.S. sent experts from the Energy and Homeland Security departments, as well as the FBI, to assist the Ukrainians in their investigation.

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