"That strange story of that former Russian spy's death got even more strange today," NBC anchor Brian Williams said.
This month, a British inquiry will investigate whether Russia is responsible. Here's what we know. (Video via Ukraine Today)
When Litvinenko left the service, he wrote this book.
Vladimir Putin was heading the FSB at the time and blamed the attacks on Chechen rebels. To this day, Russia's role in the bombings is disputed. (Video via HTB)
Litvinenko was tried for abuse of power, and in 2000 he fled Moscow for London, where he worked undercover for the U.K.'s intelligence service, MI6.
In 2006, Litvinenko met with two ex-KGB agents — Andrei Lugovoi and Dmitry Kovtun.
They met here, at London's Millennium Hotel. Hours later, Litvinenko began to vomit.
"He was able to speak, able to sign, he said, 'Putin was responsible for his death,'" Litvinenko's widow told BBC.
British police say he was poisoned with a rare and highly radioactive isotope called polonium-210 slipped into his teacup weeks before by Lugovoi and Kovtun.
"There are no grounds for speculation of this kind," Putin said. (Video via Euronews)
The public inquiry into his death is expected to reveal, for the first time, the forensic evidence police collected after his death. (Video via British Metropolitan Police)
Since 2007, U.K. prosecutors have been asking for Lugovoi and Kovtun to be extradited out of Russia, but Moscow's refused. Both men deny involvement in Litvinenko's death.