Biden holds Putin responsible for Navalny's reported death in prison.
Biden expresses outrage and condemns Putin's "thugs" for Navalny's fate.
Biden praises Navalny's role as a "powerful voice for truth."
Biden warns Putin in 2021 that Navalny's death would be devastating for Russia.
Biden states he's exploring response options but doesn't elaborate.
Biden pays tribute to Navalny's courage and achievements.
Russian officials report Navalny's death in Arctic prison.