Russia is suspected to be behind recent hacker intrusions at American power plants, including at least one nuclear facility, two U.S. officials told NBC News.
Investigators cannot definitively pin the new probing attacks, which did not affect plant controls, on Moscow. They haven't ruled out the possibility some other country's hackers, mimicking the Russians, are responsible for the breaches, the officials said.
Details of the information-collecting offensive, which began in May, were outlined in a joint bulletin by the Department of Homeland Security and the FBI. The two agencies confirmed in a statement that they had raised a red flag for the energy industry.
"There has been absolutely no operational impact to Wolf Creek," the plant, which supplies electricity for three utilities in Kansas and Missouri, said in a statement.
"The reason that is true is because the operational computer systems are completely separate from the corporate network. The safety and control systems for the nuclear reactor and other vital plant components are not connected to business networks or the internet.
"The plant continues to operate safely."
Juan Zarate, an NBC News national security analyst, said that even though the intrusions were aimed at administrative systems, they send a message to the U.S.: "Back off, because we have the ability to strike you in the heart of your core systems, your networks that matter to your economy and your lifestyle."
Last month, a group of 19 U.S. senators wrote a letter to President Donald Trump urging him to take concrete action to protect power systems from cyber-attacks.
"The Russians and other foreign actors have the capability, and potentially the intent, to cause significant damage to our economy by attacking our critical energy infrastructure, including our electrical grid," they wrote.
They asked that Trump direct the Energy Department to conduct a "thorough analysis" of Russian capabilities to threaten the energy sector and determine the extent to which the Russians have already tried to break in.
In May, Trump signed an executive order directing federal agencies to come up with a cyber-attack defense plan within 90 days.
Pete Williams is an NBC News correspondent who covers the Justice Department and the Supreme Court, based in Washington.
Robert Windrem is an investigative reporter/producer with NBC News, specializing in international security.
Tom Winter is a New York-based correspondent covering crime, courts, terrorism and financial fraud on the East Coast for the NBC News Investigative Unit.