Microsoft on Thursday fingered Russia’s military intelligence arm as the likely culprit behind ransomware attacks last month that targeted Polish and Ukrainian transportation and logistics organizations.

If the assessment by members of the Microsoft Security Threat Intelligence Center (MSTIC) is correct, it could be cause for concern for the US government and its European counterparts. Poland is a member of NATO and a staunch supporter of Ukraine in its bid to stave off an unprovoked Russian invasion. The hacking group the software company linked to the cyberattacks—known as Sandworm in wider research circles and Iridium in Redmond, Washington—is one of the world’s most talented and destructive and is widely believed to be backed by Russia’s GRU military intelligence agency.