Two prior attempts to kill ex-Russian spy Alexander Litvinenko may have been made before he died from radiation poisoning in 2006, the BBC has learned.
The one-time officer with the successor to the KGB fled to the UK where he became a fierce critic of the Kremlin and worked for security service MI6.
A public inquiry into the London death of the 43-year-old opens on Tuesday.
Mr Litvinenko's widow says the inquiry will give people "a chance to understand who killed my husband".
Marina Litvinenko says he blamed the Kremlin as he lay dying in hospital but Russia denies any involvement.
Her lawyer has described his murder as "an act of state-sponsored nuclear terrorism on the streets of London".
BBC security correspondent Gordon Corera said investigators followed a radioactive trail across London and it suggested Mr Litvinenko was poisoned not on the first attempt, but on the third.
The judge-led inquiry will be chaired by Sir Robert Owen, who was originally appointed as the coroner at Mr Litvinenko's inquest.
Sir Robert delayed the inquest and called for a public inquiry because the inquest could not consider sensitive evidence due to national security fears. The UK government resisted the move at first but later changed its stance last July, amid worsening relations with Moscow over the crisis in Ukraine.
The death of Mr Litvinenko, who took British citizenship after his arrival in the UK, had already led to a clouding of relations between London and Moscow, with expulsions of diplomats from the embassies of both countries.
He died three weeks after becoming violently ill in November 2006 following a meeting with two former Russian agents at the Millennium Hotel in central London.
UK police say radioactive polonium-210 was administered in a cup of tea, and identified two suspects in the case - Andrei Lugovoi and Dmitry Kovtun. But the two Russians have disputed their claims.
The issue of who was ultimately responsible for the death will be considered at the inquiry at the Royal Courts of Justice.
Parts of the evidence will be heard in private, and Sir Robert says it is "inevitable" that some of his final report will stay secret for security reasons.
Mrs Litvinenko told the BBC she had accepted this and trusted Sir Robert to "make the right decision".
The Litvinenko case
- 23 Nov 2006 - Litvinenko dies three weeks after having tea with former agents Andrei Lugovoi and Dmitri Kovtun in London
- 24 Nov 2006 - His death is attributed to polonium-210
- 22 May 2007 - Britain's director of public prosecutions decides Mr Lugovoi should be charged with the murder of Mr Litvinenko
- 31 May 2007 - Mr Lugovoi denies any involvement in his death but says Mr Litvinenko was a British spy
- 5 Jul 2007 - Russia officially refuses to extradite Mr Lugovoi, saying its constitution does not allow it
- May-June 2013 - Inquest into Mr Litvinenko's death delayed as coroner decides a public inquiry would be preferable, as it would be able to hear some evidence in secret
- July 2013 - Ministers rule out public inquiry
- Jan 2014 - Marina Litvinenko in High Court fight to force a public inquiry
- 11 Feb 2014 - High Court says the Home Office had been wrong to rule out an inquiry before the outcome of an inquest
- July 2014 - Public inquiry announced by Home Office
The police officer who oversaw the investigation, Peter Clarke, the former head of the Metropolitan Police Counter-Terrorism Command, says Mr Litvinenko's death was "unprecedented".
"There was a very strong forensic trail left behind because of the way - it is suggested that Litvinenko had been attacked," he said. "But what was unusual of course was having radioactivity involved. This was unprecedented."
Speaking ahead of the inquest, Mrs Litvinenko recalled her husband's deathbed claim at University College Hospital in which he said Russian President Vladimir Putin was responsible for "everything that happened to him".
It is also understood that Mr Litvinenko was visited in hospital before he died by his MI6 handler or case officer.