Explainer-Alexei Navalny's death: What do we know?

People gather outside the Russian embassy, following the death of Russian opposition leader Alexei Navalny, reported by prison authorities in Russia's Yamalo-Nenets region where he had been serving his sentence, in Warsaw, Poland, February 16, 2024. Dawid Zuchowicz/Agencja Wyborcza.pl via REUTERS

MOSCOW (Reuters) - Alexei Navalny, Russian President Vladimir Putin's most prominent domestic foe, died on Friday at the Arctic prison where he was serving a three-decade prison term.

What do we know, and not know, about the death?