Conversation so far:
How was Alexander Litvinenko poisoned?
Alexander Litvinenko's illness was later attributed to poisoning with radionuclide polonium-210 after the Health Protection Agency found significant amounts of the rare and highly toxic element in his body.
who poisoned Alexander Litvinenko?
Alexander Litvinenko met with two former agents early on the day he fell ill – Dmitry Kovtun and Andrei Lugovoy.

Question: why was he poisoned?
Self-contained rewrite: why was Alexander Litvinenko poisoned?