Why was Putin critic Alexander Litvinenko poisoned to death?

The public inquiry into the 2006 murder of former KGB and FSB officer Alexander Litvinenko began this week in the U.K. Like a story ripped from a spy novel, Litvinenko was poisoned by a lethal dose of radioactive polonium-210, administered in a cup of tea. The main suspects, two Russian ex-KGB officers, left a trail of radioactivity across London. But more than eight years after his death, the biggest question remains unanswered - why would anyone go to such lengths to kill Alexander Litvinenko? As the inquiry revealed in its first week, his poisoning was one of multiple attempts to kill the Russian dissident and outspoken critic of Vladimir Putin.

Luke Harding has been reporting on the inquiry for the Guardian and has been following the case since he was the newspaper's Moscow correspondent. He tells Brent that Putin is watching the inquiry closely and that two Russian bombers that flew over the English Channel the day after proceedings began are a clear signal from the Russian president. "Don't mess with us."

Find out what Luke thinks is the most convincing theory for why Alexander Litvienko was murdered and what other explosive testimony the inquiry heard this week.