The Litvinenko File by Martin Sixsmith, published today by Macmillan Press, goes on sale today. The Times of London offers readers an extract:

It was six o’clock on a Monday evening and the snowstorm had set in for the day. Cutting down the side of GUM, the Victorian department store that stares across Red Square to the Kremlin, I could see barely 10ft in front of me.

The red brick of the Kremlin wall emerged from the gloom and I was transported back to the first time I had come here, 20 years earlier. Then I was a young reporter with a coveted pass to attend Mikhail Gorbachev’s groundbreaking Congress of People’s Deputies, where democrats slugged it out with communist dinosaurs as Russia engaged in real political debate for the first time.

Now, in 2007, I couldn’t help wondering if much had changed. The welcome at the Spassky Gate was pretty much the same: three uniformed guards with rifles and a metal detector. But they allowed themselves a welcoming smile.

From the shadows a figure called my name. Aleksei was young, slim and cheerily informal. We chatted as we turned into the long yellow-stucco building that houses the presidential administration, the seat of power.

In the lift to the third floor I asked Aleksei who he worked for. The answer was an embarrassed: “Actually I work for the FSB; but don’t worry, I’m not a spy.”

The FSB is the Russian security service, successor to the Soviet KGB. It was about the death of a former member of the FSB that I had come to the Kremlin.

I wanted to know whether President Vladimir Putin, Russia’s most powerful leader since Joseph Stalin, had ordered the agonising death in a London hospital of Alexander Litvinenko. IT is hard to imagine that only five months ago the world had never heard of “Sasha” Litvinenko, the boy from the deep Russian provinces who rose through the ranks of the world’s most feared security service, who alleged murder and corruption in the Russian government, fled to London and took the shilling of Moscow’s avowed enemy before dying in the most sensational of circumstances last November — apparently a martyr in the covert war between the Kremlin and its political opponents.

As a habitué of Russian exile circles in London, I knew who Litvinenko was and that he was closely associated with the kingpin of the exiles, Boris Berezovsky.

Litvinenko’s second wife Marina describes him as boyish and emotional, but she says he had ruthlessness in him too. Even his closest friends say he probably had the blood of more than one victim on his hands. But he dispatched them while carrying out his duty. His constant refrain was that he had always behaved loyally and honestly.

He spent most of his career being loyal to the authorities in his country, whoever they were: first the communists, then Boris Yeltsin’s reformers, then the hardline autocracy imposed by Putin. He used to speak of Putin, a former KGB spy, as his role model, idolising him with an intensity bordering on love. But he was transformed to an acrimonious, diehard foe.

For six years Litvinenko had been venting his bile on Putin from London, hurling ever more outrageous accusations including murder and paedophilia. He had also directed increasingly bitter polemics at his former colleagues in the FSB. He had become involved in murky business dealings, with dark suggestions of blackmail plots. And he had exasperated and finally fallen out with Berezovsky himself.

The details of his death are now known worldwide. The British police have established that — in London on November 1 last year — someone persuaded him to eat or drink a dose of polonium 210, which destroyed his internal organs before doctors could discover what was killing him.

There is overwhelming evidence that Andrei Lugovoi and Dmitri Kovtun, former FSB men who met him that day, left a polonium trail all over west London. But there is no evidence that they administered the poison. Nor is there any obvious personal motive for their wanting to kill him.

Many others did have a motive for murder. In the end someone’s patience snapped. So who had the motive, and the means, to carry out what was to all intents and purposes the world’s first act of international nuclear terrorism? The answer lies in the social and political upheaval that brought Putin to power and in the business conflicts, vested interests and political corruption that have divided Russia into warring camps.

In this war each side accuses the other of the darkest acts, sometimes without the slightest basis in fact, and the hand of Putin or Berezovsky is seen behind every evil. Men like Litvinenko have been turned into the expendable pawns of ruthless masters.

As I conducted my research into his background, I was amazed by the life he had led, the risks he had taken and the enemies he had made with such insouciance. His past threw up so many potential reasons for his murder that I was surprised he had survived as long as he did.

The son of a military man, Litvinenko did his military service in an elite division under the command of the KGB and was later invited to join the KGB’s counter-intel-ligence service in the twilight years of the Soviet Union. In the mid1990s, when Chechnya was fighting for independence, he was sent there with the new FSB’s special forces, the Osobysty. He claimed to have experienced an epiphany interrogating a teenage Chechen fighter who told him: “I am not alone; the whole of my class enlisted straight after we graduated from school. We just knew we had to do it . . . for our country.”

There were also less savoury tales of his conduct. His former FSB commander in Chechnya, Lieutenant-Colonel Alexander Gusak, publicly accused him after he was poisoned last November of having been a torturer, a killer and a coward.

It is indisputable that the FSB committed many atrocities in Chechnya, and some of Litvinenko’s closest friends accept that, as a member of the feared Osobysty, he must have been involved in dirty work.

Gusak, however, is not a disinterested witness. He was intimately linked with an episode that bears directly on Litvinenko’s murder.

AFTER Chechnya, Litvinenko was invited to join a new unit, the Directorate for the Analysis and Suppression of the Activities of Criminal Organisations (URPO), set up to wipe out the crime bosses who were plundering Russia.

In Chechnya, questions of legality and human rights rarely impeded FSB operations. Now the same bespredel (lawlessness within the state) was going to be unleashed in Moscow. Litvinenko would later describe URPO as the “bureau of nonjudicial executions”.

Andrei Nekrasov, a film maker and friend of Litvinenko, told me: “That unit, to be completely frank, was composed of people that the leadership thought were capable of pulling off quite violent operations . . . and never talking about them.”

The director of the FSB at the time, Nikolai Kovalyov, says: “Litvinenko and co supported the creation of so-called White Death Brigades — in plain language, hit squads. Their reasoning was that it was impossible to combat organised crime in Russia with legal methods, so illegal methods would have to be used. That is to say, murders . . .”

In late 1997, URPO was put under the control of a senior FSB colonel, Yevgeny Khokholkov, whom Litvinenko had investigated for connections with drug gangs. Despite a compromising videotape, Khokholkov had kept his job. Disenchantment was sown in Litvinenko’s mind.

It heightened when he was ordered to ambush and beat up Mikhail Trepashkin, an FSB lieutenant-colonel who had been probing allegations that high-ranking officers were involved in serious crime.

Unable to defend himself, the slightly built Trepashkin begged Litvinenko for the chance to explain what he had found out about FSB corruption. Marina Litvinenko says he convinced her husband that things were badly wrong in the FSB and that someone had to do something about it.

The chance to do something soon arose. On December 27 1997, according to Litvinenko, he and four other URPO officers were called into Khokholkov’s office and told to assassinate Boris Berezovsky.

No written order was given. There was clearly a well-established process of deniability in place: such decisions were taken in cosy chats on sofas in private offices with no minutes and no paper trail. This is of crucial importance now in examining the decision-making process behind Litvinenko’s own assassination nearly a decade later.

In 1997 Berezovsky was probably the most powerful man in Russia. He and other postcommunist billion-aires had rescued Yeltsin from defeat in the 1996 presidential elections with unlimited money and media support. In return, Yeltsin had rewarded them with the keys to Russia’s economy, auctioning off state companies at knockdown prices.

Berezovsky was also a media magnate. His real interest, however, was the acquisition of power. He exerted such influence over the weak and chronically drunk president that he was widely regarded as making decisions for him. By the time Litvinenko was ordered to kill him, everyone knew that Berezovsky was a man not to be trifled with.

Litvinenko had an additional problem: he knew Berezovsky well. He had investigated a bomb attack on the rising tycoon in 1994, and they had become friends. The relationship had been cemented when, Berezovsky says, Litvinenko prevented the Moscow police from framing him for the murder of a prominent television presenter. “Alexander really saved my life, there was no doubt about it.”

For two months, Litvinenko and his comrades carefully teased out who was behind the proposed assassination, talking to contacts and sources, trying to discover if its backers were themselves powerful people and whether or not it would be in their own interests to go along with it. They knew a bad call could mean an end to their careers and, quite possibly, their lives.

Concluding that the top people in the FSB didn’t know about the order to kill Berezovsky, they reported it to the director. The move backfired. Khokholkov denied their story, and they were put under investigation.

Meeting secretly, the five men decided to seek protection from their proposed victim. Berezovsky could be a very powerful patron for a group of ambitious FSB officers looking to further their careers. Litvinenko told him the whole story.

“Initially I thought it was just a joke,” says Berezovsky. But he also spotted the potential to get control of the FSB.

He asked Litvinenko to bring the other four men to his office to make a videotape of their allegations. Only three turned up, but on the video one is heard quoting the order they received: “He said to us, ‘If there was an order to knock someone off — sorry, to kill; he said to kill — could you fix it?”

Berezovsky: “To kill me?” Agent: “Yes, of course you.” An FSB man later identified as Alexander Gusak also describes on the tape a face-to-face meeting with Khokholkov where he was asked if he would kill Berezovsky. “I replied that if it was properly sanctioned and had the right stamps — that is, the stamp of the prosecutor’s office and the stamp of our own organisation — and it had the right materials to back it up, I would be ready to kill Berezovsky and anyone else.”

Berezovsky took the incriminating videotape to a rising star in the Kremlin: Vladimir Putin, at the time a presidential aide. Berezovsky considered him a reformer and a friend. They regularly visited each other’s houses and even took skiing holidays together.

At first the ploy seemed to work. Putin took charge of the FSB, and the hated Khokholkov was transferred. Litvinenko thought he would have a big role in a cleaned-up FSB under his hero Putin.

Berezovsky had helped get Putin appointed and now expected him to pay this favour back by installing friendly faces in all the positions of power. If things worked out, the FSB would become a loyal Berezovsky fiefdom for him in the looming power battles over the succession to Yeltsin.

It didn’t work out, however. Putin’s debt of gratitude was small beer compared with the need to look after number one. The Berezovsky camp was just one among several warring Kremlin factions he weighed up to decide where his best interests lay.

To apply pressure on Putin, Berezovsky told Litvinenko and his colleagues to go public with their revelations about the assassination plot in a televised press conference. When some of the shocked agents refused — it was unheard of for FSB men to go public — he told them they had come too far to turn back.

On the eve of the press conference he summoned them to a grey-stuccoed building that had once been the family mansion of the noble Smirnov family. Inside, they were served drinks in Berezovsky’s club, the Logovaz Salon, with its gilded walls, ornate decorations and giant aquarium. Then they were coached on the statements they would be making.

Next day, in front of the cameras, Litvinenko accused his superiors of extortion, kidnappings and murder and, in a not very coded message to Putin, called on the FSB to cleanse itself.

Litvinenko identified himself but the five men with him were not so brave: one wore a ski mask and the others dark glasses. I now believe I know their names, which would recur with ominous regularity in both Litvinenko’s future life and the investigation of his eventual death.

They included Gusak, who would accuse Litvinenko of war crimes in Chechnya; Colonel Viktor Shebalin, who sat next to Litvinenko making an exaggerated show of friendship and support; and Major Andrei Ponkin, who was the only other man to speak. Ponkin alleged, among other things, that he and others had been instructed to kill the dissident former FSB man Mikhail Trepashkin.

Far from being nudged into cooperating, Putin was infuriated. The whistle-blowers were called in by FSB interrogators. Some were threatened, others offered inducements. It was made forcefully clear to them that they had brought shame on the service and the motherland. They could face the prospect of prison, or they could recant and agree to work against the “traitors” who had led them astray.

The question of exactly which of Litvinenko’s comrades succumbed to these blandishments is a vital piece of information for anyone seeking to unravel the events that led to his death.

Marina Litvinenko claims Shebalin was working all along for the FSB as “a provocateur”. Litvinenko’s friend, the historian Yuri Felshtinsky, believes Shebalin, Ponkin and Gusak all took roles in Putin’s subsequent war against Berezovsky and Litvinenko.

From that day on they would have every incentive to silence the increasingly irritating voice of the man they claimed had tricked them into putting their lives and careers on the line.

Events then moved rapidly. Berezovsky slid down the greasy pole of Kremlin politics as Putin rose up it. He was given a ceremonial job that kept him out of Moscow, and in March 1999 he was ousted altogether. Within days Litvinenko was arrested for trumped up petty crimes.

Felshtinsky says that the FSB tried to persuade Litvinenko to cut a deal in the same way that his former colleagues seem to have done. “When he refused . . . the FSB told him, ‘Look. Now you must know the end of the story. The end of the story is that you are going to be killed, or you are going to be put in prison and killed in prison. But you know our organisation: there is no other way. You are going to be killed’.”

He was charged with beating up an arrested terror suspect. To seasoned FSB men this was ridiculous; few could think when arrested terror suspects were not beaten up.

The prosecution produced a grainy video of a blond FSB officer punching a crouching prisoner in the face. The interrogator is wearing a military cap and I certainly could not identify him as being Sasha Litvinenko. He is, however, surrounded by apparent URPO agents.

Two officers who had served with Litvinenko recognised the film and they knew the man in it was not him. They found the original of the tape, which had other footage proving his innocence.

According to Litvinenko’s father, Walter, they were about to produce the tape in court when Litvinenko was threatened in his cell. “The FSB came to him and said, ‘You have a son. If you produce that video in court, you should be very afraid for your son’.”

Even without the video, the military judge threw out the case. But as he did so a team of crack spetsnaz troops — Russia’s SAS — stormed into court and the military prosecutor announced new charges. After another acquittal a third trial was ordered, and harassment continued.

FSB interrogators warned Litvinenko: “If they find you not guilty this time, it’s not you we’ll be talking to; we’ll sort things out with your wife and your kid. You don’t think you’ll get away, do you? You’re a traitor to the system and you’re going to be punished.”

His friend Felshtinksy made an unofficial approach to an FSB general, asking if a deal could perhaps be done for the Litvinenkos to slip quietly into exile abroad. The general replied, “I can honestly tell you there is no way for that man to leave Russia alive. And if ever I meet him again, I will personally kill him with my two hands.”

That, says Marina Litvinenko, is when Sasha made up his mind to flee, as Berezovsky had done. During his personal drama Yeltsin had resigned and Putin was now president.

SEVEN years later, at the end of a wide Kremlin corridor, Aleksei ushered me through an anteroom into the large office of Dmitry Peskov, Putin’s head of information.

Peskov is a sprightly man in his early forties, a career diplomat who enjoys the confidence of the president. Over a cup of hot Georgian tea I tried to gauge if his boss really could have been involved in the Litvinenko poisoning, as the dead man’s friends have claimed, or if the accusations were merely the fabrications or wishful thinking of enemies at home and abroad.

An earnest, sophisticated man, Peskov is far removed from the bullying, stonewalling Soviet officials I used to meet. He comes across as reasonable and sincere in his love for his country and his faith in his president. He knows Putin intimately — he works with him every day — and feels personal resentment on behalf of his boss.

I knew he had discussed the Litvinenko case with Putin at great length and had given him advice on how to remain calm and measured in the face of what the president believed to be an unjustified personal affront against himself.

“You know, I would never discuss that [advice] in public. But nevertheless, what is obvious is that the president felt himself necessary to express his condolences to the family of Litvinenko. He accepted that it was a human tragedy — a man died — but he never tried to camouflage, to hide the fact that he was not fond of Mr Litvinenko. And you will find very few people in my country — including his first wife, by the way, and his two children — who are fond of him or who are proud of him. This is not the case in my country.”

It was a strange sensation, sitting in the heart of the Kremlin discussing the personal feelings of the most powerful man in Russia. Would previous occupants of these quarters have been so open with a foreigner?

I asked how Putin felt about the allegations levelled at him personally, how it felt to be accused of murder. Peskov said he would not discuss such things in public, but I later spoke to another source close to Putin who knew about his feelings.

“The president is very upset by this,” he told me. “He is upset by these accusations made personally about him. He simply can’t believe that people are saying these things about him as a person. He’s very angry about the way the British press has named him as a murderer — that’s why he won’t speak about it any more.”

I asked why, if this was the case, Putin had refrained from expressing his anger and hurt. He told me: “The president doesn’t like his feelings being discussed in public.”

Even if Putin had not personally ordered the Litvinenko killing, it could still have been the unauthorised work of the Russian security services. I asked Peskov if the president had ordered an inquiry to make sure the FSB was not involved.

“Look, I don’t know. I am being very frank with you now. It’s not a question of Putin not being sure if such an involvement was possible or impossible. It is hard for us to imagine that there is the slightest idea that such a possibility could exist. For us the tiniest possibility is out of the question. There is not even the tiniest possibility, not even a hypothetical possibility of our special services being involved.”

Up to now I had been convinced by what I had heard. On the balance of evidence I was coming to the conclusion that Putin himself had had no hand in the murder. But this was something different: Peskov could offer no evidence that ruled out the possibility of a freelance operation, or that suggested Moscow had even tried to rule one out.

When I pressed him he told me: “For that purpose our prosecutor’s office has opened its own investigation.” It was clear where I would have to go.

The office of the prosecutor general of the Russian Federation is set behind a small, anonymous-look-ing wooden door on Bolshaya Dmitrovka, a street behind the Bolshoi theatre. The prosecutor’s office is a powerful institution, combining oversight of policing, investigation and prosecution. The Metropolitan police’s finest visited it two weeks before me, looking for clues in the Litvinenko case.

As in the Kremlin, my reception was warm and friendly. Two young detectives, Sasha and Kolya, walked me upstairs to a cosy, overheated second-floor office. An attractive woman in her mid-thirties introduced herself as Marina Gridneva, senior legal counsel and head of the information division. She introduced another detective, and they produced a teapot and a large sponge cake topped with apricot jam. It was, explained Marina, homemade. With a cup of a very unusual, aromatic tea, I ate two slices.

The charm offensive seemed genuine and they laughed when I said journalists would not get similar treatment from Scotland Yard. But hospitality did not mean they were going to answer my questions. All my inquiries about the possibility of FSB involvement in Litvinenko’s murder were met with a steely: “That is part of an ongoing investigation so we cannot comment.”

After 20 minutes we seemed to be getting nowhere. I decided to be a little provocative. “What about the new laws of July 2006?” I asked. One of them allows the president to use the Russian secret services to eliminate “extremists” in Russia and on foreign territory. And another expands the definition of “extrem-ism” to include anyone “libellously critical of the Russian authorities”?

“It looks like a pretty clear mandate to go out and kill people like Litvinenko, doesn’t it?” I suggested.

The two detectives asked for a moment to consult. They tapped at a computer and phoned for some documents. My tape recorder registered an air of mild panic. Marina’s voice is heard asking me to help myself to some more tea and cake while they sort things out. Then, after a lengthy pause, they are back with the explanation: those laws were not adopted with any evil intent. They were a response to the abduction and murder of five Russian diplomats in Iraq.

It seemed I was going to get nowhere. They had stonewalled me with a charming but immovable double act. So I said, “Okay, thanks very much”, and they clearly thought the interview was over because they started smiling and suddenly became very expansive. Fortunately, my tape was still running to record what came next.

“Look, Martin, do you really think we’d bother assassinating a nobody like Litvinenko? Someone who left the country God knows how long ago? Who was no threat to us and didn’t have any secrets to betray? . . . He just wasn’t important enough. He didn’t know any secrets that would be a reason for liquidating him . . . Do you think we would have mounted such a special operation to eliminate him . . . with polonium that costs the earth? That we would have spent so much money on him? My God, we could have used the money to increase pensions here at home. If we’d needed to eliminate Litvinenko, we would have done it ages ago.”

I thanked them and switched off the tape recorder. It was the closest I was going to get to an admission that such operations do after all take place. And if they take place, was it not possible that someone had his own reasons to conclude that Litvinenko actually was worth the price of a vial of polonium?

The more I probed, the more I was becoming convinced that Litvinenko had been poisoned by a group of people independent of the Kremlin but with close connections to the Russian security forces.

I knew it was a group with its own reasons to target Litvinenko, a group that could advance FSB interests to justify the murder, interests that would confer at least some immunity on it if the Kremlin were to become aware of what it had done.

How was I to find these men? I sat in my Moscow hotel room, a half empty bottle of vodka in front of me, and picked up the telephone.