After months of extraordinary testimony the public inquiry into the murder of Alexander Litvinenko resumes on Friday. One of the two alleged killers, Dmitry Kovtun, is due next week to give evidence by video from Moscow.



Here are three things we’ve learned and three things we still don’t know.

Three things we know ...

The identity of the murderers

The public inquiry into Alexander Litvinenko’s murder began in January and finally concludes next week. It’s been exhaustive: 70 witnesses, 15,000 pages of evidence, months of hearings in the high court in London.



One thing is clear: two Russians, Dmitry Kovtun and Andrei Lugovoi, murdered Litvinenko by slipping radioactive polonium into his green tea. The forensic evidence against them is enormous. The killers left a trail. Scotland Yard’s radiation schedule – it runs to a whopping 265 pages – is a ghoulish and fascinating document. Scientists tested everything including a bronze phallus in a nightclub visited by Kovtun and Lugovoi during their last London trip (it was negative).



On Friday, the final witnesses are likely to give more proof of the pair’s guilt. They include former colleagues of Kovtun from the days when he worked as a waiter at an Italian restaurant in Hamburg. Kovtun told one of them – “D3” – he was planning to kill a “traitor” using a “very expensive poison”. He even asked D3 if they knew a cook in London who might put the poison in Litvinenko’s food or drink.

Kovtun and Lugovoi are safe in Moscow having refused to travel to the UK. In March, just as the inquiry was wrapping up, Kovtun said he was willing to testify as a “core participant”. He is scheduled to give evidence by video on Monday, Tuesday and Wednesday. Kovtun is likely to claim that Litvinenko poisoned him. This is nonsense. Tests show that Litvinenko had no contact with polonium prior to his meeting with the two men from Moscow, who had already left alpha radiation in their hotel bedrooms.



The inquiry has established a motive

Litvinenko lived in Britain for six years before his poisoning. In exile, he was a remorseless Putin critic. He made speeches, wrote articles, and co-authored a controversial book, Blowing Up Russia, which accused Putin of orchestrating a series of apartment block bombings in which nearly 300 people died. The bombings, Litvinenko alleged, gave Putin a pretext to invade the rebel republic of Chechnya. The book came out in 2002. A stream of anti-Putin allegations followed. So why murder him in 2006?

The evidence laid before the inquiry offers a persuasive answer. Litvinenko was killed because of his investigation into the Russian mafia and its links with prominent Kremlin figures, including Putin himself. The mafia took root in Spain from the mid-90s onwards. Litvinenko helped Spanish intelligence in operations to “behead” top mafia bosses.

From hundreds of wiretaps, as well as bank documents, and money transfers, Spanish investigators found these bosses had intimate links with senior Kremlin politicians. They came to a stunning view: that Russia had, in effect, become a “mafia state”, a phrase used by Ben Emmerson QC, counsel for Marina Litvinenko, on the first day of the inquiry. At the time of his murder, Litvinenko was due to be a star witness in a series of trials.



One Russian in the frame is Viktor Ivanov, a powerful Putin ally, who heads the Kremlin’s federal narcotics service. In September 2006, Litvinenko wrote a report alleging that Ivanov was part of a criminal network in St Petersburg and had links in the 1990s with Colombian drug cartels. At the time, Putin worked in St Petersburg’s mayor’s office. Litvinenko wrote: “While Ivanov was cooperating with gangsters, he was protected by Vladimir Putin ... who was not Mr Clean at that time.”

Litvinenko gave a copy of the the report to Andrei Lugovoi. He thought Lugovoi was his business partner, but actually Lugovoi was flying to London to kill him. Weeks later Litvinenko lay dead.



Putin has thumbed his nose at the inquiry

A day after Emmerson described Russia as a “mafia state” Russian Tupulov bear bombers buzzed the south coast of England. Coincidence? No, rather a typical sabre-rattling sign of Kremlin displeasure. Downing Street was forced to scramble two RAF typhoons to track the aircraft. The following month, Russian bombers turned up near the coast of Cornwall.

But Putin’s most demonstrative act was to give a major state honour to Lugovoi, now an influential deputy in Russia’s Duma. On day 22 of the inquiry, Interfax announced that Lugovoi was getting a medal for “services to the motherland”. Supposedly this was for his work in parliament. In reality, it was a thank you to Lugovoi from the top and a sign that he still enjoys warm presidential support.



And three things we don’t know …

The secret evidence

Over the last few months the inquiry has been hearing secret government evidence. We don’t know details. The hearings have not been public. But it is bound to include MI6’s classified file on Litvinenko, who worked part-time for the British intelligence agency from 2003 until his murder in November 2006. He was an informant, giving expert advice on Russian organised crime across Europe. Did MI6 know that its source was at risk? And if not, why not?



The material may also feature phone transcripts and emails gathered from British and US interception operations against prominent Russian targets. (GCHQ’s eavesdropping capabilities are widely known, post-Snowden, but not officially acknowledged.) Plus, reports from MI6 agents in the field. And the foreign office’s internal conclusion as to who murdered him – the Russian state. It was this classified report and Putin’s refusal to hand over the killers that in 2007 led the then foreign secretary David Miliband to expel four Russian diplomats from London.

Only Sir Robert Owen, the inquiry chairman, has been allowed to review these documents. They will inform his conclusions but will be redacted from his final report to home secretary Theresa May, due later in 2015.



We know where the polonium came from but are missing details

The inquiry was told the polonium used to kill Litvinenko came from Russia. It started off as bismuth, and was irradiated at the Mayak nuclear complex in the Urals region. It was then transported to Avangard, a closed government facility in the city of Sarov. Avangard is the only laboratory in the world that produces polonium-210 on a commercial line. (The UK stopped making it in the 1960s.)



After this the trail goes murky. According to Professor Norman Dombey, a physicist, a “government institution” would have converted the polonium into soluble form. Dombey means the FSB, the Russian spy agency. Its predecessor the KGB ran its own notorious poisons factory, which Lenin set up in 1917. We don’t know who gave the polonium to Lugovoi and Kovtun in Moscow, possibly at the airport. It was almost certainly concealed in a purpose-built vial. (The amount was tiny. Litvinenko drank just 26.5 micrograms.)

We do know that it took them three attempts – one bungled, another aborted – before they finally managed to put it into Litvinenko’s tea on 1 November 2006, at the Millennium hotel. It was an expensive hit. The cost was “tens of millions of dollars”, Emmerson said.



Who gave the order to kill Litvinenko?

Several witnesses argue that only Vladimir Putin could have given the order to liquidate Litvinenko, their friend. Yuri Shvets, who co-wrote Blowing Up Russia, says that in Soviet times assassinations were agreed at the highest bureaucratic level – not least because Kremlin officials were keen to cover their backs. Alex Goldfarb characterised Putin’s conflict with Litvinenko as personal and said it’s inconceivable a group of rogue officers could have murdered him without the boss’s say so.

All of this is plausible. But it doesn’t amount to proof. Soviet leaders traditionally gave only verbal instructions when it came to state murders: there was no paper trail. They talked in oblique terms; Stalin preferred the indirect phrase “action”. In interviews with detectives, given hours before his death and revealed at the inquiry, Litvinenko said the truth about his poisoning would emerge when the current Putin regime collapsed. Or its spy chief defected to the west.



Neither of these things has happened yet. The full truth is there but we may have to wait for it.



Luke Harding is the author of Mafia State: How One Reporter Became an Enemy of the Brutal New Russia, published by Guardian Faber