WHO HAS IT KILLED?

Polonium poisoning is so rare that it took doctors several weeks to diagnose Litvinenko's illness and security experts struggled to think of a previous case. More than five years after Litvinenko's death, no one has been arrested. British prosecutors have named ex-KGB agent Andrei Lugovoi as their chief suspect, but Russia refuses to hand him over.

Some speculate that the Curies' daughter Irene, who died of leukemia, may have developed the disease after accidentally being exposed to polonium in the laboratory.