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MIAMI (CBSMiami) — The FBI and Homeland Security Department issued a new warning to American energy companies about potential cyberattacks on nuclear facilities.

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The warning says hackers working for a foreign government have been able to find vulnerabilities in companies that operate nuclear plants. Russian cybercriminals are suspected.

The frequency and intensity of these cyberattacks have escalated recently and that is what prompted federal officials to issue the warning late last month.

Among the companies targeted by hackers is the Wolf Creek Nuclear operating corporation in Kansas.

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U.S. officials said the hackers were able to penetrate the business side of the nuclear facility. Officials insist that there was no threat to public safety or to operations at the plant.

Last month the FBI and department of Homeland Security issued a joint report warning businesses to watch out for specific types of cyberattack attempts by hackers.

There was a similar report on Russian malicious cyber activity in December.

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In the latest incidents, officially, it is too early to attribute the attacks to a specific country. However, former officials familiar with Russian cyber capabilities said they have to be at or close to the top of the list of potential suspects.