Litvinenko died Nov. 23 in a London hospital after ingesting radioactive polonium-210.

NPR: Kremlin Issues Warning After Expulsions

He was a friend and close associate of Alexander Litvinenko, the former Russian security agent who died in London in 2006 after drinking tea laced with radioactive polonium.

WSJ: Boris Berezovsky, Exiled Russian Tycoon, Dies

British officials are still puzzled by the use of this poison, the radioactive isotope Polonium 210.

NPR: Radioactive Poison as an Assassin's Tool

Mr Litvinenko died in 2006 after he was apparently poisoned with the radioactive substance polonium-210.

BBC: Alexander Litvinenko murder was 'London nuclear terror'

Mr Berezovsky was a close friend of murdered Russian emigre and former KGB officer Alexander Litvinenko, who died in 2006 after he was poisoned with the radioactive material polonium-210 while drinking tea at a London meeting.

BBC: UK

Alexander Litvinenko, a renegade Russian security officer living in London, was killed by poisoning with polonium, a rare radioactive substance, in 2006.

ECONOMIST: Assassinations and technology

On that day Litvinenko fell sick, suffering (it eventually turned out) the effects of poisoning by polonium, a rare radioactive substance that killed him three weeks later.

ECONOMIST: British prosecutors name their man