(Bloomberg) — North American utilities are scouring their systems for signs of Russian malware that the U.S. government has warned could give hackers control of water treatment facilities and parts of the electrical grid.

The U.S. Department of Homeland Security issued alerts about digital attacks on utility computer systems on Oct. 8, Oct. 17 and Oct. 28. The agency didn't identify the country behind the hacks, but cybersecurity firms yesterday connected them to Russia. The firms have cautioned in recent reports that cyberspying by Russia is on the rise, and a recent breach of an unclassified White House computer system was linked to the Russian government or criminal hackers.

The DHS alerts said malware called BlackEnergy was used to access to human-machine interfaces, systems utility operators use to control critical functions. U.S. investigators haven't detected attempts to modify or damage those systems, according to the Oct. 28 warning, suggesting that infiltrators were trying to gain control for later use.