There is a fascinating legend that Alexander I faked his own death and "retired" from life under another name.
At one point a man named Fedor Kuzmich appeared in Tomsk and local residents suspected that this might well be Tsar Alexander himself. In 1864 Kuzmich was lying on his death bed, his last words being, "God only knows my real name!"
Allegendly, in 1865, Tsar Nicholas attempted to quell the rumors about his brother and had his casket opened. Rumors soon spread about the casket being empty. Then in 1926, over one hundred years after Alexander's death his casket was opened again and was officially declared empty. To this day, supposedly no one really knows where he lays buried.
I would forget about the Soviet government's announcement in 1926. It was almost certainly disinformation, one of many such lies intended to discredit Russia's former rulers by making them seem like fools, tyrants, or bizarre individuals of one sort or another.
The Soviet's 1926 statement about Alexander's coffin has never been backed up by any kind of evidence.
I don't think that professional historians anywhere give any credence to the empty coffin story. It was communist proganda, plain and simple.
As for the Kuzmich story, it is true that such a peasant lived in Siberia in the mid-1800s. He was remarkable for knowing things about the Russian imperial court that a simple peasant wouldn't be expected to know. Still, knowledgable people who interviewed Kuzmich came away convinced that he was an ignorant peasant and nothing more.
The documentation and eyewitness accounts surrounding Alexander's death in Taganrog in 1825 are very credible. You can rest assured that Alexander died there, just as historians say.
Alexander died of an illness that was either malaria or something akin malaria. The disease was rampant in the Taganrog area at the time.