U.S. Federal prosecutors say three Russian spies allegedly tried to take over energy plants as part of a vast hacking conspiracy that targeted hundreds of energy companies in 135 countries, including some in the United States.

The three men allegedly were a part of a secret unit in the Russian Federal Security Service called “Dragonfly,” the U.S. Department of Justice announced on March 24.

The indictment alleges Pavel Aleksandrovich Akulov, 36, Mikhail Mikhailovich Gavrilov, 42, and Marat Valeryevich Tyukov, 39, used “spearphishing” email scams to gain access to various companies’ systems, then hid malware in software updates distributed to over 17,000 users.

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The two-phased plot reportedly took place between 2012 and 2017, with targets including the U.S. Nuclear Regulatory Commission, an unnamed renewable energy facility in New York, and Wolf Creek Nuclear Operating Corp. in Kansas, where the spies were indicted in August a grand jury.

“Russian state-sponsored hackers pose a serious and persistent threat to critical infrastructure both in the United States and around the world,” Deputy Attorney General Lisa O. Monaco said. “Although the criminal charges unsealed today reflect past activity, they make crystal clear the urgent ongoing need for American businesses to harden their defenses and remain vigilant.”

The indictments were made public as Russia’s ongoing invasion of Ukraine further isolates Ukraine diplomatically. As Russia feels the pinch of economic sanctions imposed due to the war, the U.S. has warned that large-scale cyberattacks are a possibility, Fox News reports.

“The potential of cyberattacks to disrupt, if not paralyze, the delivery of critical energy services to hospitals, homes, businesses, and other locations essential to sustaining our communities is a reality in today’s world,” U.S. Attorney Duston Slinkard said. “We must acknowledge there are individuals actively seeking to wreak havoc on our nation’s vital infrastructure system, and we must remain vigilant in our effort to thwart such attacks.”

The DOJ is offering a reward of up to $10 million for information leading to the arrest of a defendant or identification of other conspirators in this case as part of its Rewards for Justice program.