Former officer of the Russian FSB secret service Alexander Litvinenko died on the 23rd of November 2006 in London. He had been an outspoken critic of the Russian government and made several allegations particularly concerning the death of Russian business oligarch Boris Berezovsky and the Russian government's involvement.
Litvinenko died of lethal polonium-210-induced acute radiation syndrome, an extremely rare and puzzling way to die -- infact, Litvinenko is known to be the only person to have died this way. Previous radiation syndrome victims included Hiroshima and Nagasaki and nuclear reactor accident survivors, and of course Marie Curie who discovered the element.
The only question is how he acquired it.
Polonium, being a radioactive element, can be traced and someone poisoned with it can leave a harmless trail that can remain for a good while in the locations someone carrying it was in. As such it was possible to trail certain locations Litvinenko had visited by the polonium remains, as well as people he had come in contact with. However, polonium poisoning is only deadly in high amounts and relatively harmless on passing contact and as such he must have ingested it.
Knowing of his death, Litvinenko made a death-bed accusation directed at Vladimir Putin, naming him as the one who had given the order for his murder. A few former KGB agents Litvinenko had met the day before he fell ill also showed traces of polonium and an ongoing investigation by Scotland Yard has announced that "the evidence suggests that the only credible explanation is in one way or another the Russian state is involved in Litvinenko’s murder".
While there is no definitive proof that the Russian government is to blame, there seems to be no doubt that he was purposefully given the polonium. A few theories suggest he may have been smuggling polonium as it is a very lucrative business and may so have come in contact with the material, but the evidence for this is not strong enough and unless he swallowed the element to smuggle it it would not cause such a lethal reaction.
So while there is no definitive proof that someone deliberately poisoned him, all evidence does suggest foul-play by one party or another and Litvinenko is as such considered to have been the victim of an assasination.