Hackers working for a foreign government recently breached at least a dozen U.S. power plants, including the Wolf Creek nuclear facility in Kansas, according to current and former U.S. officials, sparking concerns the attackers were searching for vulnerabilities in the electrical grid.

The enemy could be positioning itself to eventually disrupt the nation’s power supply, warned the officials, who noted that a general alert was distributed to utilities a week ago. Adding to those concerns, hackers recently infiltrated an unidentified company that makes control systems for equipment used in the power industry, an attack that officials believe may be related.

The chief suspect is Russia, according to three people familiar with the continuing effort to eject the hackers from the computer networks. One of those networks belongs to an aging nuclear generating facility known as Wolf Creek — owned by Westar Energy Inc., Great Plains Energy Inc. and Kansas Electric Power Cooperative Inc. — on a lake shore near Burlington, Kansas.

The possibility of a Russia connection is particularly worrisome, former and current officials say, because Russian hackers have previously taken down parts of the electrical grid in Ukraine and appear to be testing increasingly advanced tools to disrupt power supplies.

The hacks come as international tensions have flared over U.S. intelligence agencies’ conclusion that Russia tried to influence the 2016 presidential election. The U.S., which has several continuing investigations into Russia’s activities, is known to possess digital weapons capable of disrupting the electricity grids of rival nations.

It was unclear whether President Donald Trump was planning to address the cyberattacks at his meeting on Friday with Russian President Vladimir Putin. In an earlier speech in Warsaw, Trump called out Russia’s “destabilizing activities” and urged the country to join “the community of responsible nations.”

Representatives of the National Security Council, the Director of National Intelligence, the Department of Homeland Security and the Nuclear Regulatory Commission declined to comment.