The U.S. has charged four Russians over their attacks on the energy sector between 2012 and 2018.

The four Russian works were accused of allegedly installing backdoors and launching malicious software designed to compromise the safety of energy facilities.

The indictment also linked three of the hackers to Military Unit 71330, or “Centre 16” of the FSB. FSB is Russia’s security service.

Some of their cyber activities include carrying out two separate emergency shutdowns at one facility in Saudi Arabia, and an alleged attempt to hack a company that managed critical infrastructures in the US.

“Russian state-sponsored hackers pose a serious and persistent threat to critical infrastructure both in the United States and around the world. 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,” US Deputy Attorney General Lisa Monaco said.