I had heard vaguely that it was believed by some that Stalin, who officially died of a stroke in 1953, may have died as a result of exposure to Polonium in his clothing.
Is there a definitive answer for this mystery?
To my mind it isn't even a theory anymore, he was almost certainly poisoned. The physical evidence is that he suffered stomach hemorrhaging, which was excluded from the initial report and is inconsistent with suffering a stroke (Faria). Beria was alleged to have claimed he poisoned Stalin as well (New York Times).
I seem to remember there being a convincing argument for it having been warfarin poisoning, but don't remember the source. Actually there was a better source than Faria iirc it was a modern scientific review of his autopsy.
Sources:
https://www.nytimes.com/2003/03/05/world/new-study-supports-idea-stalin-was-poisoned.html
Yes.
The new declassified material about his death and the way he died points to the fact that he was indeed poisoned. Most probably by Beria at least indirectly.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3228382/
"We now possess clinical and forensic evidence supporting the long-held suspicion that Stalin was indeed poisoned by members of his own inner circle, most likely Lavrenti Beria, and perhaps even Khrushchev, all of whom feared for their lives.[4] But Stalin, the brutal Soviet dictator, was (and still is in some quarters of Democratic Russia) worshipped as a demigod — and his assassination would have been unacceptable to the Russian populace. So it was kept a secret until now."
People like Djilas though Stalin was "senile" when he met him in later life, so he probably could no longer have control over everything.
Beria boasted to Molotov "I did him in. I have saved you all!" (probably correctly so) but soon it was his turn...
And since Beria was executed pretty quickly a lot of secrets were buried too.... Beria's archive as well as that of NKVD was also destroyed by Khruschev after his death (you can read this in Zhores and Roy Medvedev's book Unknown Stalin which I strongly recommend). And most of Beria's men was killed too (well except Pavel Sudoplatov who wrote his Memoirs much later, in English" Special Tasks, the Memoirs of a Soviet Spymaster" with a lot of interesting information).
So unfortunately we will never know the real truth about Stalinist Soviet and especially its last years, including the death of Stalin himself.