Navalny’s death at age 47 has deprived the Russian opposition of its most well-known and inspiring politician less than a month before an election that will give President Vladimir Putin another six years in power.

Although neither the imprisoned anti-corruption crusader nor other Kremlin critics were in position to challenge Putin for the presidency, the loss of Navalny was a crushing blow to Russians who had pinned their future hopes on Putin’s seemingly indefatigable foe. It also prompted questions about what killed him.

A note handed to Navalny’s mother stated that he died at 2:17 p.m. local time Friday, according to Navalny spokesperson Kira Yarmysh. Prison officials told his mother when she arrived at his former penal colony Saturday that her son had perished due to “sudden death syndrome,” Ivan Zhdanov, the director of Navalny’s Anti-Corruption Foundation, wrote on X, formerly Twitter.