The United States on Thursday imposed new sanctions on four Russian security agents it accused of being involved in the 2020 poisoning of imprisoned opposition leader Alexey Navalny.

The Treasury Department said the four Russians are linked to Moscow’s Federal Security Service. Treasury officials blocked any U.S. assets they may hold and prohibited them from conducting U.S. financial transactions, while the State Department ended the possibility of visa rights to enter the U.S.

In a statement, Treasury anti-terrorism official Brian Nelson said, “Today we remind Vladimir Putin and his regime that there are consequences not only for waging a brutal and unprovoked war against Ukraine, but also for violating the human rights of the Russian people.

“The assassination attempt against Alexey Navalny in 2020 represents the Kremlin’s contempt for human rights, and we will continue to use the authorities at our disposal to hold the Kremlin’s willing would-be executioners to account,” Nelson said.

Navalny, a longtime Putin critic, was detained in January 2021 after returning to Moscow from Germany where Western doctors had treated him for what they said was poisoning by a Soviet-era nerve agent.

The Kremlin at one point accused Navalny of working with the CIA to undermine Russia. Moscow has denied any involvement in what happened to him and denies persecuting Navalny.

Navalny, 47, has become a target of Putin’s crackdown on dissent. Navalny had been serving a nine-year sentence on fraud and embezzlement charges but was recently sentenced to serve another 19 years on extremism-related charges.

The Russian agents were identified as Alexey Alexandrovich Alexandrov, Konstantin Kudryavtsev, Ivan Vladimirovich Osipov and Vladimir Alexandrovich Panyaev.

Some material in this report came from Reuters.