On Dec. 29 and 30, the Polish electricity grid was subjected to a cyberattack that nearly knocked out power to hundreds of thousands of households.

Security firm ESET has since conducted a closer examination of the attack and concluded that it was carried out by Sandworm, a group of hackers with strong ties to the Russian military intelligence service GRU.

The attack used Dynowiper, a powerful malware that deletes all data on vulnerable computers.