What cases are you thinking about?

Even the Wikipedia article says that the correct diagnosis — the poisoning by polonium-210 — was made to a large extent due to a happy coincidence: there happened to be a scientist in the facility where Litvinenko was treated who happened to have worked with polonium. Were it not for that coincidence, it is very possible the correct diagnosis would not have been reached, and the very investigation that followed would not gone very far.

So I am not sure how Litvinenko poisoning is an illustration of an assassination attempt advertised as such to the rest of the world. On the contrary, it looks like a murder attempt that was intended to have never been solved.

Seriously? Using Polonium meant that it took weeks for Litvinenko to die, giving him plenty of time to make it clear exactly who was responsible, and even give interviews to the press. If they wanted to keep it quiet, they did a pretty terrible job.