Dunaev was a member of a cybercriminal organization that deployed a computer banking trojan and ransomware suite of malware known as "Trickbot", the Justice Department said.
Believed by researchers to be a Russian-speaking group of hackers, cl0p was recently able to gain access to a wide swathe of organizations' data by compromising MOVEit Transfer
Investigators are considering a number of theories, from someone who misplaced the documents to an insider who actively wanted to undermine US national security interests.
The Russian embassy in Washington and the Kremlin did not respond to requests for comment.
Justice Department spokespeople declined to give a comment to the WSJ Saturday on the investigation.
The documents appear to have been altered to lower the number of casualties suffered by Russian forces, the US officials said.
The lawmaker urged the public to stay vigilant after being targeted with a fake mail, purportedly sent by a staff member.
Moscow is attempting to make its hacking sprees as psychologically difficult for Ukrainians as possible.
Royal Mail unable to despatch items to overseas destinations as of Thursday because of a Russia-linked ransomware group
Several US-based nuclear labs were targeted, such as Brookhaven (BNL), Argonne (ANL) and Lawrence Livermore National Laboratories (LLNL).
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