Who killed Litvinenko?

The first hearing in a public inquiry into the 2006 murder of ex-Russian secret agent Alexander Litvinenko is being held on Tuesday in London's Royal Courts of Justice.

On November 23, 2006 the former FSB agent, who had fled to Britain in 2000 and been granted UK citizenship, died of radioactive polonium-210 poisoning in a hospital in London aged 43. On his deathbed, he said he believed that Russia's President Vladmir Putin was involved in the attack on him.

The police investigation led to the conclusion that Litvinenko drank a cup of tea laced with the poison on November 1, when in the company of Andrei Lugovoy and Dmitri Kovtun, both ex-Russian secret agents. Lugovoy has since become a lawmaker in his native Russia.

Roughly a year later, the British prosecutors named Lugovoy and Kovtun chief suspects in the case, but Russia refused to extradite them, saying that its constitution does not allow it to extradite its own citizens.

The Litvinenko affair led to a low point in British-Russian relations, which British Prime Minister David Cameron has tried to overcome since taking power in 2010. The crisis in Ukraine has soured relations further.

Long road from inquest to inquiry

The UK's Coroner's Act from 1988 requires a coroner to hold an inquest when someone dies a violent or unnatural death, though it is purely a fact-finding procedure and does not apportion blame.

In Litvinenko's case, the inquest was adjourned for nearly five years while the possibility of criminal proceedings was being investigated. By October 2011 the inquest resumed when it became clear that no criminal prosecution would be brought as the two main suspects reside outside the UK and were not being extradited by Russia.

The government initially refused to call a full inquiry as it would mean examining documents considered "sensitive to the highest degree," and could therefore affect national security.

But after a High Court ruling called for the government to reconsider the decision, Britain's Home Office announced on July 22, 2014 that there would be an inquiry into Litvinenko's death, and that the inquest was suspended. Robert Owen, a senior judge who had also conducted the inquest, is chairing the inquiry.

Litvinenko's wife Marina had been pushing for an inquiry for years - "to know the truth, for my right to know what happened to my husband," she told Reuters news agency.

'Fair and fearless investigation'

When he formally opened the inquiry on July 31, 2014, Owen spoke of "regrettable delays" during the inquest.

Owen had long spoken out in favor of a public inquiry as a more appropriate way to conduct a "fair and fearless" investigation that would tackle the question of whether the Russian state had a role in Litvinenko's murder.

Owen said there was a "prima facie case as to the culpability of the Russian state," meaning that sufficient corroborating evidence appears to exist to support a case. The inquiry allows the court to examine sensitive material in private.

Russia has refused to participate in the inquiry. The Foreign Ministry in Moscow said last September that "it is hard to imagine how such a process could be objective," referring to the fact that some evidence will be heard behind closed doors.