Court 73 in London's Royal Courts of Justice has pale-wood panels, a suspended ceiling, stackable chairs, lever-arch files. It is dull, almost aggressively so. Yet if these off-white walls could talk, what tales they'd tell: of Tony Blair humbled by Lord Hutton; of Rupert Murdoch summoned to Lord Leveson; of the deaths of Diana, Dodi, David Kelly, Mark Duggan and the 52 victims of 7/7.

Court 73 is where enquiries, inquiries and inquests answer our biggest questions. It's where incendiary details are wrested from British institutions. And, on 27 January 2015, a new question was asked, addressing perhaps the most explosive secret yet: did Vladimir Putin bottle radioactive death, dispatch assassins to London and poison a Russian dissident in the heart of the British capital?

It's a question with the potential to destroy diplomacy, to make Putin's pariahdom permanent, to relegate Russia to rogue status in perpetuity. Just asking it forces the West to rethink its post-Cold War complacency. It is about as big a geopolitical question as you can get and the answer will depend on one man: the coroner Sir Robert Owen. Sir Robert is a retired High Court judge, jowly and solid. Here he was, justice's champion, sitting at the high dais in Court 73. It was a few seconds after 10am.

The air was thick, clogged with concentration. Lines of journalists looked over lines of lawyers. Everyone focused on the slight silhouette of Marina Litvinenko on the left-hand side of the front row. She watched Sir Robert patiently as he cleared his throat, her short, brown-blonde hair swept back at the temples, her jawline angled towards him above the collar of an embroidered silk shirt. Her sweater was black; more than eight years after her husband's murder, she was still in mourning.

"Today we begin the open hearings in the inquiry into the death of Alexander Litvinenko," Sir Robert said, hitting his consonants hard for emphasis. "The issues to which his death gives rise are of the utmost gravity."

The public inquiry was set up by home secretary Theresa May to ascertain how, when and where Litvinenko came by his end and where responsibilty lies. This is the story of that inquiry. If this were a film - and it could be one, with its spies, its crooks, its plots, its hypocrisy, its naivety, Litvinenko's agonising death, the poison that killed him, his beautiful, tenacious wife - this would be the moment for the courtroom to fade into flashback.

Alexander Litvinenko was born on 4 December 1962 in the provincial Russian city of Voronezh. He died on 23 November 2006 in London. His birth passed unnoticed outside his family; his death made headlines around the world. An ordinary man strayed into high politics and died for it, most terribly.

Brought up by his grandparents, Litvinenko inherited the unquestioning Soviet patriotism born of victory in the Second World War. He went straight from school to the army and then, in 1988, to the KGB. It was a natural path for a patriot to take. After 1991, he moved to the body that came to be called the FSB, and was tasked with trying to understand the gangsters who were spreading through the ruins of Communism like rats in a shattered city. Litvinenko came to suspect his commanding officers were in cahoots with this new mafia.

In 1997, he claimed, they ordered him to kill Boris Berezovsky, a mathematician turned oligarch. Berezovsky engaged in ceaseless power struggles and, it appears, someone in the FSB had got fed up. Litvinenko believed the order to be illegal and took his worries to the FSB's director. The director brushed him off, so Litvinenko warned Berezovsky, who put him on television. Media coverage was feverish, Litvinenko's colleagues furious.

The FSB's director sacked the rogue agent. Multiple criminal cases were opened against him. The director, meanwhile, rose fast, becoming prime minister, then president. His name was Vladimir Putin. Litvinenko was looking at a lifetime of prosecution. In November 2000, he, Marina and their son, Anatoly, fled, claiming asylum at Heathrow.

Berezovsky fled too, setting up a rival court in London. With time, he came to play the same role in Putin's propaganda that Leon Trotsky played for Joseph Stalin: the libertine exiled Jew versus the Kremlin's spartan Christian. Litvinenko loved Russia and dreamt of serving it. Now he was abroad, working for the Kremlin's enemy.

Marina did not give evidence until the inquiry's third day, but her narrow-shouldered figure dominated the courtroom regardless. Dr Nathaniel Cary, the pathologist who opened up Litvinenko's radioactive body ("One of the most dangerous postmortem examinations ever undertaken," he said), kept glancing towards her as he spoke of tissue samples, of organ failure, of the precautions he and his team had taken to prevent the poison harming them too.

"My role was to recover what was by any standards a very hazardous body," he stated, before stuttering to a halt.

Sir Robert intervened: "I would quite understand if Mrs Litvinenko would not wish to be present during the..." Her lawyer, Ben Emmerson QC - cropped hair, cockneyfied speech, cocksure - cut him off. His client would be present throughout the evidence, he said. No one tried to patronise her again.

She remained dry-eyed even when a scientist itemised the contamination of each part of her husband's body. When the list reached his testicles, she looked down for a couple of beats, her eyes closed. But then she raised her head and squared her shoulders again.

I watched her shoulders, because I couldn't see much of her face. Indeed, for the spectators, the whole process was something of a ballet of backs: barristers stood up, scratched themselves, pushed hands down the waistbands of their trousers. A stretch in the suit fabric between buttock and shoulder suggested they were preparing a point, leaning into a question. Only Sir Robert faced the room, and his expressions were unreadable.

On day three, a Monday, Marina was already sitting in the box as the public filed in. She had an interpreter, but rarely needed her. She spoke for herself, the imperfections of her English enriching the strength of her testimony: "did" instead of "yes"; "not" for "no"; "w" for "v".

It took her all day to talk through her 13 years of life with Litvinenko, their meeting, their lives together in Moscow, their flight from Russia, their concerns in exile. She looked tiny and fragile in the witness box, watchful as a wren. The first of November 2006 was the sixth anniversary of their arrival in London, she said, and the first such anniversary they would spend as British citizens. Litvinenko came home after a day of meetings and she cooked chicken pancakes, the way her mother had cooked them, the way he liked them. He ate five. It was a celebration.

"He said he feels, he said he felt sick. It was very sudden... and he said, of course he couldn't say anything, he just vomited," she said, looking down now. "First one, it was exactly what he just ate for dinner. Then after that, it became, looked like water, even more not food, with very strange colour."

The Millennium Hotel in Grosvenor Square, London, where Litvinenko drank a fatal dose of polonium-210 on 1 November 2006 © Rex Features

He had been poisoned with polonium-210, a rare and highly radioactive isotope. And it was flooding his liver, his kidneys, his bone marrow. It was emitting countless alpha particles, helium nuclei stripped of their electrons, atomic artillery. They were slamming into his cells, smashing his DNA, extinguishing his bodily processes. There is no antidote; he was already dead; his body just hadn't realised it yet. The symptoms - vomiting and diarrhoea; collapsing numbers of blood cells; hair loss; constant pain - were like nothing the doctors knew. Three weeks passed and they still weren't sure what was wrong. "It was like all pain, his body was just absolutely difficult to do, to feel anything, because it was just pain," Marina said, her grammar breaking apart. When she left the hospital that evening of 22 November, to go home to their son, she felt guilty, promised to be back in the morning. He whispered that he loved her and she tried to make a joke out of it, teased him for not telling her often enough, promised him everything would be fine.

Litvinenko had had fluffy blond hair, a heavy brow. He was chunky, 40-something, athletic. By the day of his death, he was Lord Voldemort: flat-bald, his jaw defined, his face pale, his eyes slitted. He was pronounced dead at 9.21pm on 23 November: the first confirmed victim of a nuclear attack since Nagasaki. "I was sitting to his bed and I could touch, I could kiss him, and nobody said it might be dangerous or I shouldn't do this," Marina said. Even now, she didn't cry.

When Litvinenko arrived in Britain, London's relations with Moscow were close. Tony Blair had worked hard to cosy up to Putin, impressed by his efforts to stabilise Russia, untroubled by his squalid, horrible war in Chechnya. Putin was a man Britain could do business with. Litvinenko wanted to disabuse the West of that notion, of any idea that Putin was a force for good. He resolved to expose the links between Russia's mafia and top officials. He co-wrote a book with another émigré called Blowing Up Russia, which accused the FSB of "acts of terror, abductions and contract killings". I've had a copy since 2003, though I'd never actually opened it until I started writing this article. The trouble with Litvinenko was that his accusations were so wild that hardly anyone paid attention.

His hatred for Putin was visceral. And he accused the president of anything, even of being a paedophile. It was easier for journalists to dismiss his allegations altogether than to sift through for ones that checked out. His death has given his claims more weight than they held in life but, nonetheless, as the inquiry went on, most witnesses admitted they, too, had struggled with his politics.

"I found him a bit crazy," said one. "A bit tiresome... everything was a conspiracy," said another. "He was a bright chap but... mentally undisciplined," said a third. A fourth compared him to "a radio station talking all day long". Even Berezovsky agreed in a police interview: "He'd say things that were too silly to be true."

Robin Tam QC, barrister for the inquiry, asked Alex Goldfarb, a Berezovsky-funded activist who befriended Litvinenko, if he also thought the ex-KGB man was an obsessive, a conspiracy theorist: "Well, at the time I thought so, but with what has happened since, I became a conspiracy theorist myself." Just because you're a conspiracy theorist, it doesn't mean someone isn't planning to poison you with a deadly radioactive isotope.

Litvinenko was hungry for information and sought out Soviet dissidents, Chechen rebels, Italian mafia experts, anyone who could give him a new perspective. He continued to expose corruption, but there wasn't much money in it. And that was a worry, particularly in 2006 when Berezovsky cut his allowance. So Litvinenko found work by writing reports on Russia's officials. Private intelligence is a murky world, shading into the security services on one side and organised crime on the other. He sought out sources among his old friends. Among them was Andrei Lugovoi, a blond businessman with the perpetual pout particular to moneyed Muscovites.

Lugovoi was an ex-KGB man too and had worked for Berezovsky back in the Nineties before branching out on his own. He organised corporate security, was doing well, wore jewellery and ill-fitting suits. He was the kind of Russian that Londoners laugh at, but not to his face and only after they've accepted his cash.

Lugovoi visited Litvinenko at home and they chatted regularly on the phone. On 16 October 2006, they met to discuss a contract at a security company in London, together with a friend of Lugovoi's. The friend was Dmitry Kovtun, a drifter whose serial attempts to launch a career (army officer, porn star, nude model, fixer, trader) over the previous 15 years had foundered on the rock of his incompetence. There were glasses of water on the table.

That night, Litvinenko vomited, though the sickness did not last. He had survived a first attempt to kill him.

The three men met again on 1 November, at the Pine Bar of the Millennium Hotel in Grosvenor Square. Lugovoi was going to a football match - CSKA Moscow were playing Arsenal - and pre-loading on whisky and gin. CCTV captured him and Kovtun going to the toilet, Lugovoi in a canary-yellow sweater and grey slacks, Kovtun in a black rollneck. Cameras on the streets outside caught flashes of Litvinenko hurrying to his doom, wearing a denim jacket with a light collar, cellphone to his ear.

Litvinenko, a teetotaller, ordered nothing, but accepted a cup of tea from a pot already on the table. The tea was almost cold, he remembered in the days before his death, and he only had a few swallows. The meeting was quick. Lugovoi's family was waiting to go to the game, including his eight-year-old son, Igor.

"This is Uncle Sasha, shake his hand," Lugovoi said, and Igor solemnly greeted the man his dad had just killed. Litvinenko went home to eat that lovingly prepared chicken. Lugovoi headed to the Emirates to watch a scoreless draw.

The Metropolitan Police has charged Lugovoi and Kovtun with murder. They deny it and have come up with a wide range of alternative explanations: Litvinenko killed himself to discredit Putin; he killed himself by accident while handling polonium; he was blackmailing Berezovsky, who killed him; and so on.

Neither of the men is sufficiently confident in the plausibility of any of these versions to return to Britain to face trial, and Russia has refused to extradite them. One of the Kremlin's pet opposition parties gave Lugovoi a seat in parliament in 2007 and thus immunity from prosecution.

The inquiry heard how Lugovoi even taunted Berezovsky, sending him a T-shirt in 2010. On the front it bore the words, "Polonium-210, London, Hamburg, to be continued", along with the emblem of the CSKA football club. On the back it said: "Nuclear death is knocking on your door."

The question of whether Kovtun and Lugovoi killed Litvinenko is something that, of course, can only be decided by a jury. However, were I to be a member of that jury, I wouldn't bother even walking out of court before reaching a decision. Kovtun and Lugovoi are murderers and we know this thanks to the one witness that can't lie: polonium-210.

Only one place makes polonium; it's called Avangard and it's in Russia. Polonium's half-life is 138 days, so it cannot be stored for long. The dose that killed Litvinenko must have been made shortly before and made in Russia. That dose didn't have to be large, however. Polonium is hundreds of thousands of times more poisonous than hydrogen cyanide: the lethal dose is in millionths of a gram, a fraction of a grain of salt. Such tiny quantities can only be detected if you're already looking for them.

Litvinenko’s widow, Marina, forced a reluctant UK government to hold an inquiry into his death © Getty Images

It would be the perfect poison, except for one thing: polonium turns the poisoner into a nuclear King Midas, irradiating everything he touches. Officers checked every hotel room used by Kovtun and Lugovoi, every car, every nightspot. The men left a trail like cats' feet in wet concrete. The polonium contaminated a hubble-bubble in a West End bar, a teddy bear in Kovtun's ex-wife's house, a passport photograph. Officers could see which tables the men used in restaurants, which seats they used in planes, which box Lugovoi used at the Emirates.

In the Met's graphics, light contamination was coloured green. Wide swathes of hotel rooms were green, as were restaurants, beds and cars. Higher contamination was coloured yellow, higher still red. The most radioactive areas were purple, and they told an eloquent tale. There was purple in the bin of Lugovoi's hotel room and purple in the U-bend of his sink. The table in the boardroom where the men met on 16 October had a purple spot where Litvinenko's glass had been. The table in the Pine Bar was purple where the teapot sat and on the side of Litvinenko's chair. There was purple inside the spout of the teapot and under the rim of the lid.

"In my mind, there isn't a shadow of a doubt that it is the teapot used to poison Mr Litvinenko," stated Detective Inspector Craig Mascall. A teapot: these Russians couldn't have picked a more English murder weapon.

The general impression was that Kovtun and Lugovoi splashed polonium around like Old Spice. They were so careless with their deadly liquid that it seems impossible that they knew what it was. And, surely, whoever gave them such an incriminating poison never imagined it would be identified. In fact, its identification was something of a miracle. Because polonium emits primarily alpha radiation, which does not penetrate the skin. Once it was inside Litvinenko's body, it was almost undetectable. Only an inspired hunch from a professor led to a sample of Litvinenko's urine being sent to a nuclear lab and thus to the poison being identified. That was just a few hours before he died.

Evidence was fed to the inquiry slowly and in diluted form. Attention wandered and, by the third or fourth week, the ranks of public seats were almost empty. Perhaps because of the relentless grimness or just the experience of sitting in the same room every day, those of us who remained grew close.

Sir Robert Owen took to joking a little with witnesses and developed a rapport with some of the regular attendees: the policemen, the translators, the lawyers. The ushers picked up on his lead and swapped gossip during breaks. By the end of February, coming to a hearing felt like going to school, as if this had been going on forever.

If it felt age-long for the attending media, it's hard to imagine how it must have felt for Marina, who has been battling for justice since almost before Litvinenko died. The diplomatic repercussions of the murder had been immediately clear. "This case is obviously causing tension with the Russians. They are too important for us to fall out with," a minister (unnamed) in Blair's government told the Sunday Times on 3 December 2006, four days before the ex-KGB man's lead-lined coffin had even been interred.

However, Russia's failure to do much to help the Met and refusal to extradite the suspects inevitably resulted in a cooling of the warmth between Putin and Blair. There has also been a growing realisation that Litvinenko was essentially correct: Putin's government is an enrichment machine for insiders, not a serious attempt to make Russia better for the Russians. That was not something that bothered the post-Labour government much, and David Cameron visited Moscow in 2011, seeking to boost trade. The fact Russia was shielding the men accused of murdering Litvinenko was annoying, sure, but business is business.

In the circumstances, Marina's relentless insistence that her husband's murder be investigated was inconvenient. If she got her inquiry and if it ruled that Putin had, in effect, launched a small nuclear attack on London; that could prove, well, awkward. That was a risk Whitehall did not want to take and it fought Marina at every step.

In 2012, Berezovsky cut off financial support for her legal fees, leaving her potentially liable for the government's costs. She continued. After the High Court ruled no inquiry was necessary, she burst into tears on its steps, and appealed to the people of Britain: a widow out of Shakespeare, abandoned in an unjust world. But she was not alone; among those who saw the need for an inquiry was Sir Robert. He wanted to fulfil his duty of investigating Litvinenko's death and to do so publicly. When Marina's lawyer said she was "extremely disappointed" by a further delay at a hearing in early 2013, Sir Robert went further: "Not as disappointed as me," he replied.

Marina beat the government on appeal and last year it finally surrendered, probably because - post-Crimea - there was little relationship with Russia that was worth saving. Who now would expend political capital protecting Putin from a righteous widow? Sir Robert and Marina got an inquiry.

It was not quite the inquiry they'd wanted, however. Firstly, several key witnesses - not least Berezovsky, who hanged himself in March 2013 after losing a lawsuit to Roman Abramovich - were unavailable. Secondly, the British government managed to exclude from the inquiry's terms of reference any examination of its own failure to protect Litvinenko and also ensured much of the evidence would be heard in secret. The 34 days of open hearings were just the visible spectrum of the light being shed on this murder.

For some witnesses, journalists had to leave the court and watch on video link. Other witnesses were neither shown nor identified. A scientist called A1 gave days' worth of information on polonium, which we effectively had to hear on a radio. A witness, known as C2, did the same, and recounted a bizarre subplot in which Kovtun attempted to recruit him to poison Litvinenko's food via a group of fellow émigrés the Russian had known while working as a waiter in Hamburg in the Nineties.

C2's friends had their own code names - D3, D4, D5, etc - something that caused him great confusion. It was hard to blame him for his bafflement; someone in the court delighted in overcomplicating things. They had assigned his wife a code name - C3 - apparently just for the hell of it. She had no connection to the case at all.

At least it was clear what these restrictions were. Other restrictions only became visible when one of the lawyers blundered into them. They had to agree all questions with Sir Robert in advance and he sometimes intervened to block them from asking something.

The concern, inevitably, was "national security". Litvinenko had, it transpired, worked for MI6 and the Spanish secret services. What he did for MI6, how long he'd been doing it, who he did it with... about all of these things we have no idea. Any time the inquiry strayed near, Sir Robert headed it off. Perhaps he is learning about them in closed session while I type. Perhaps he isn't. That knowledge is secret, too.

"I shall perform a global analysis of the evidence adduced both in the open and the closed hearings," said Sir Robert, in summing up Day 29. "The consequence of the restriction orders that have been made mean that parts will not be published if to do so would be to damage the national security or international relations. I hope that makes the position clear."

Robin Tam, QC: "Sir, yes. Sir, indeed."

Litvinenko was an unusual murder victim, not just in the way he was killed, but in the time he took to die. He was able to tell the police about his meetings with Lugovoi and Kovtun, as well as his other activities and thus essentially to become a witness to his own assassination. He also had time to write a postmortem statement.

"You may succeed in silencing one man but the howl of protest from around the world will reverberate, Mr Putin, in your ears," said the statement, read by Alex Goldfarb outside University College Hospital on the morning of 24 November 2006. "May God forgive you for what you have done, not only to me but to beloved Russia."

The levels of polonium contamination at 25 Grosvenor Street, London, where two Russian agents tried to poison Litvinenko on 16 October 2006. Red indicates strong contamination © Rex Features

The levels of polonium contamination at 25 Grosvenor Street, London, where two Russian agents tried to poison Litvinenko on 16 October 2006. Red indicates strong contamination

It is one thing to accuse Putin of murder, but another to explain why he would arm two amateurs with one of the world's deadliest substances and dispatch them to a world capital. The inquiry heard a string of possible motives for this apparent recklessness. A Chechen said Putin was angry with Litvinenko's Chechen work; an Italian said Putin was angry with Litvinenko's Italian work; Berezovsky had said it was because of his work with Berezovsky. In short, no one knew.

It is clear, however, that Putin has no interest in having the murder solved. When the British and German police asked to test specific Russian planes for radiation, those planes never arrived. When British police visited Moscow in late 2006, they met Lugovoi and Kovtun, but may as well not have done. Their meeting with Kovtun lasted just 13 minutes. Only one British officer was allowed to attend (but not to speak), and their Russian counterparts asked only a handful of the Met's 118 questions.

Their meeting with Lugovoi was longer but, when the Russian police came to hand over the recording, their Dictaphone had malfunctioned and no tape would be forthcoming. "To say it was unfortunate would be an understatement," said Clive Timmons, a detective superintendent at the time.

The British officer who did attend had made notes of the discussion and was able to compare them to the transcript the Russians provided. They were identical, except in one passage, in which Lugovoi described how Litvinenko had helped Spain identify organised criminals. Litvinenko, it appears, told the Spanish the same story he was telling his corporate clients: the mafia and the FSB were the same thing. Russian police deleted this information before handing Lugovoi's transcript to the Brits. Someone in Moscow did not want anyone in London to know about it.

So who in particular was angry about Litvinenko's work? And who didn't want the truth about Russia's mafia to reach foreign ears? In 2006, Litvinenko was contracted to write a report on Viktor Ivanov, another ex-KGB agent, who had worked with Putin for more than a decade. (Ivanov was then deputy head of the Kremlin administration. Since 2008, he has led Russia's Federal Drug Control Service.) Litvinenko subcontracted two others to help him: an American, Yuri Shvets and Lugovoi.

Lugovoi's contribution was short and useless. Shvets' contribution was long and informative, detailing allegations that Ivanov was tied to organised crime, money laundering and drug trafficking. Litvinenko used it in full and forwarded it to Lugovoi to show him what a report was supposed to look like. That was in late September. The first poisoning attempt was less than three weeks later; Litvinenko was dead within two months.

Shvets said, "It is my understanding that as a result of this report the deal didn't go through and Mr Ivanov didn't get the kickbacks."

"Kickbacks of what sort of order?"

"From what I understand... between $10 million [£6.6m] and $15m [£9.9m]."

So, we have a motive: Lugovoi passed the report to Ivanov, who realised Litvinenko's work was costing the Kremlin lots of money. Thus, he had to die.

Is that what happened? It goes without saying that Russian officials, and Ivanov himself, deny it, although their denials are infuriatingly inconsistent. A statement from Russia's London Embassy on 6 August this year was the latest to add to the confusion. It speculated that Britain deliberately gave visas to Lugovoi and Kovtun so as to frame them and hinted that ties with Russia were being sacrificed to save MI6 from embarrassment.

"There cannot be too much openness and transparency in this case. Only truth, all the truth and due process could help overcome this artificial irritant in [the] Russo-British relationship," the statement said.

It remains unclear why, if Russia wants due process and transparency, it will not extradite the suspects, or try them itself. One reason, perhaps, is that, according to the inquiry's lawyers, the secret evidence is enough to make a "prima facie" case that the Kremlin killed Litvinenko. (This means that, were Russia a person, the evidence would be strong enough to justify charging it with murder.) Sir Robert is presumably now going through that evidence and will put his considerable weight behind the punches it allows him to throw next month. But, does Putin even care?

Since the Ukraine crisis began we have learnt about Russia's "troll factories", offices full of Russians deployed to neuter bad publicity by spamming the internet. (Read the comments on any Russia-related article on the Guardian website.) As the world has become less convinced by the Kremlin's angle on world events, it has expanded this philosophy offline: to diplomacy, military co-operation, trade and, now, to Sir Robert's inquiry.

First, Russia said Sir Robert could use material gathered by Russian detectives, then it said he couldn't. First Kovtun said he would give evidence by video link, then he said he wouldn't. These communications came in answers to letters sent months earlier and were received in July, when Sir Robert was finishing up. They were clearly intended to sabotage the proceedings as thoroughly as possible. After Kovtun's final failure to appear by video link, Sir Robert's face was closer to brick red than its usual pink. "I will not allow my duty to investigate to be subverted," he stated, his voice deadly calm.

He could hardly have doubted Putin's desire to do so, however. On 9 March, the inquiry's Day 21, the Kremlin awarded Lugovoi the medal "Services to the Fatherland", which is given for "a great contribution to the defence of the Fatherland... for the preservation of state security". It was the presidential equivalent of inking up a middle finger, rolling it on a postcard and addressing it to Sir Robert Owen, c/o Court 73, the Royal Courts of Justice.

Sir Robert's reply is due in next month. It will be more carefully worded, and it needs to be, for he holds the future of our relationship with Russia in his hands. What British politician would ever befriend Putin again if Sir Robert rules him a nuclear terrorist? And would such a ruling be the spark to push Putin into some form of political retaliation? The world can only watch and wait - on high alert.