British authorities are now investigating the death of Alexander Litvinenko, a former Russian spy killed in London by radiation poisoning in 2006. The mystery long surrounding his demise heightened Thursday when a lawyer representing Litvinenko’s widow said he was a “registered and paid” agent for the British MI6. The Russian government is suspected in his death.

In 2006, Litvinenko, 43, fatally ingested tea laced with the rare radioactive isotope polonium-210, according to the Washington Post. Russian authorities have denied any involvement, but Litvinenko’s family has claimed he accused President Vladimir Putin by name while on his deathbed.

Litvinenko worked for the Russian FSB, the successor of the KGB overseas intelligence, and rose to the rank of colonel before breaking ranks by publicly refusing an order to assassinate exiled Russian tycoon Boris Berezovsky. Litvinenko fled to Britain during the late 1990s and wrote a book in 2003 accusing the Russian government of organizing bloody bombings in Moscow that instigated the second Chechen war.

The New York Times reported in 2006 that Roger Cox, director of the Health Agency’s Protection Agency’s center for radiation, said polonium-210 is present in the human body at a low level and is only dangerous if someone eats or drinks the chemical.

“If that enters the body by ingestion, then it will rapidly track through the body and go to most major organs,” he said.

Polonium is a nuclear byproduct and was also found at a bar Litvinenko frequented, his home and at a hotel Litvinenko visited briefly. He died about a week after his health rapidly started declining and pictures were taken of Litvinenko after he had lost considerable body weight and all of his hair.

Before he was killed, the former Russian spy was investigating the Russian mafia, but his lawyers now suspect the very men who were supposed to be helping Litvinenko in orchestrating his death. One of the men is know a high-ranking Russian lawmaker.

Despite the diplomatic tension that has stemmed from the case, Thursday’s inquiry was encouraging for Marina Litvinenko. The spy’s widow told reporters, “I appreciate all that was done today and I’m looking forward to any decision which will be taken by the coroner after today’s hearing.”