Lugovoi, a millionaire with a background in the private security business, had met with Litvinenko in London on Nov. 1, hours before the former KGB operative said he felt ill from poisoning by radioactive substance polonium-210.

FORBES: Magazine Article

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