WASHINGTON (NewsNation) — President Joe Biden Thursday vowed to sanction Russian President Vladimir Putin in response to opposition leader Alexei Navalny’s death.

Biden made the announcement following a meeting with Navalny’s widow, Yulia Navalnaya, and daughter, Dasha Navalnaya, in California.

“We are going to announce sanctions against Putin, who is responsible for [Navalny’s] death, tomorrow,” the president told reporters. “We are not letting up.”

Navalny, one of the Kremlin’s most ardent critics, died in an Arctic penal colony last week while serving a 19-year sentence on charges that his supporters have categorized as “politically motivated.” The cause of death is still unknown.

In a readout of Biden’s meeting with Navalny’s family, the White House said the president conveyed “his heartfelt condolences” for their loss.

Biden also “expressed his admiration for Aleksey Navalny’s extraordinary courage and his legacy of fighting against corruption and for a free and democratic Russia in which the rule of law applies equally to everyone.”

The president emphasized that Navalny’s legacy will “carry on through people across Russia and around the world mourning his loss and fighting for freedom, democracy, and human rights.”

Washington seeks to hold Moscow accountable for the sudden death of Navalny and its full-scale invasion of Ukraine. 

The U.S. and its allies have already imposed thousands of sanctions targeting Russian elites and oligarchs who have supported Putin’s war.