I have a little from Radzinsky
Alexander II the Last Great Tsar page 22
"In Tagnrog, the emperor[Alexander I] died swiftly and unexpectedly. The official diagnosis has been preserved, but it is so vague that it is difficult to guess what disease killed Napoleon's conqueror. The rumor that reached Moscow right after the news of his death has also survived the centuries; Alexander I did not die. Another corpse was in the coffin, and the emperor went off to be a hermit in Siberia, to pray and repent for his terrible sin against his father [Paul I].
The arrival of the coffin bolstered the rumor, since the coffin was not opened when the emperor lay in state. This was the first time that the court said its farewells to a ruler without seeing his face. Even Peter III and Paul I, who had marks of violence upon them, were laid out to be seen.
The court was told that the heat in Taganrog had caused the body to decay. But everyone knew that the body had been embalmed. People repeated the strange words of Prince Volkonsky, that "the Emperor's face, despite the embalming, had turned black and even the features had changed completely."
Of course in 1894, the same thing happened to Alexander III and that coffin was left open and everyone was required to kiss the portrait that he held in his hands. It was said that his face was black and that the stench was overwhelming.
Who knows