The Biden administration unveiled sanctions Tuesday against seven senior Russian officials over the poisoning and detention of Alexei Navalny, the most prominent domestic critic of Russian President Vladimir Putin.

Senior administration officials told reporters on Tuesday that the US intelligence community had concluded with “high confidence” that officers of Russia’s Federal Security Service, known as the FSB, poisoned Navalny with the nerve agent Novichok. The use of Novichok agents is barred in warfare under the Chemical Weapons Convention of 1997, of which Russia is a signatory. Navalny is not the first Russian dissident to be poisoned with Novichok, a class of nerve agents first developed by the Soviet Union during the Cold War.

“Russian officials have targeted Mr. Navalny for his activism and efforts to reveal uncomfortable truths about Russian officials’ corruption,” an administration official said. “We’re exercising our authorities to send a clear signal that Russia’s use of chemical weapons and violation of its international human rights commitments have severe consequences.”

Read the in-depth SOFREP report on the Russian Protests

The officials reiterated Biden’s demand that Navalny be released from prison, saying his arrest and detention occurred on “politically motivated grounds.” Navalny was accused of repeatedly violating the terms of his 2014 parole by not reporting to authorities in person, and this included while he was receiving treatment abroad after his poisoning.