“Sudden death syndrome” is an umbrella term for many different causes of cardiac arrest.

The location of Navalny’s body is unclear. A morgue near the Arctic prison where he died said it had not received the body, according to his team.

They were later told by officials that the body would not be handed over until an investigation is complete — even though his lawyers were previously informed that the investigation “had been concluded and that something criminal had not been established,” according to his spokesperson Kira Yarmysh.

Navalny was arrested and jailed in 2021 after returning to Russia from Germany, where he successfully recovered from nerve agent poisoning in what his allies believed was a Kremlin-ordered murder attempt.