Officials say the Russians obtained access to the energy system and "conducted network reconnaissance" of industrial control systems that run U.S. factories and the electricity grid.
U.S. national security officials say the FBI, the Homeland Security Department and intelligence agencies have determined that Russian intelligence and others are behind attacks on the American energy sector.
The U.S. officials say the energy industry targets were chosen deliberately, according to the Star Tribune.
The officials say the Russians obtained access to the energy system and "conducted network reconnaissance" of industrial control systems that run U.S. factories and the electricity grid. The officials say they've helped companies kick the Russians out of all systems currently known to have been penetrated.
The U.S. is sending an alert to energy companies to raise threat awareness.
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The Homeland Security Department says a Russian government hacking operation has affected the U.S. nuclear, aviation, water, construction and manufacturing sectors.
The department's U.S. Computer Emergency Readiness Team says the operation targeted smaller American commercial facilities. The operation is said to have resorted to various methods — including a kind of cyberattack known as spear-phishing — to try to compromise legitimate user accounts, gather user credentials, and target industrial control systems and their networks.
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The alert says the FBI and the National Cybersecurity and Communications Integration Center have determined that the ultimate objective was to "compromise organizational networks."