A series of cyberattacks has been leveled at the computer networks of American nuclear power plants.

The New York Times reveals that the Department of Homeland Security and the Federal Bureau of Investigation have reported a number of hacks targeting companies that run U.S. energy facilities over the past two months, including the Wolf Creek nuclear plant in Kansas.

The attacks appear to have used a variety of techniques to gain access to computer systems. They include phishing attacks, which sent malicious code inside Microsoft Word documents via e-mail, along with so-called watering hole attacks, which compromise websites known to be used by targeted staff members.

So far, it appears that hackers gained access to office computer systems but failed to take control of devices that are used to operate of any of the facilities. So right now, the hacks are worrying rather than dangerous. The reports do, however, suggest that the hackers may have been using the opportunity to map networks, which could be used as intelligence for future attacks.

It’s important to not overstate the threat posed by the hacks, which hit administrative systems rather than industrial control computers. It’s unclear what the motive behind the attacks was, but they nonetheless raise fears that hackers could at some point in the future shut down parts of the American energy grid, or, less likely, undermine safety systems on nuclear facilities.

The news adds to concerns raised by recent attacks on energy infrastructure in Ukraine, which were considered by many to be test runs for taking down facilities in other countries. Research suggests that some of the malware tools developed by hackers and used against Ukrainian systems could prove to be a potent threat to industrial systems around the world, though no link has yet been drawn to the new American hacks.

Currently it’s unclear who is responsible for the hacks. But according to three people familiar with the attacks that spoke to Bloomberg, “the chief suspect is Russia.” The Times, meanwhile, points out that the DHS and FBI say that it was carried out by an “advanced persistent threat”—which is usually shorthand for a hacking collective that’s supported by a nation state.

(Read more: New York Times, Bloomberg, “Ukraine’s Power Grid Gets Hacked Again, a Worrying Sign for Infrastructure Attacks,” “A Hack Used to Plunge Ukraine Into Darkness Could Still Do Way More Damage”)