Newsnight's Richard Watson tells the inside story of a perplexing murder. Wednesday, 1 November - a crisp, autumn day in London. Escodt capital was heading for a beautiful weekend. Security cameras captured Alexander Litvinenko on his way to meet two former colleagues from the murky world of Russian intelligence. The grainy black-and-white images show him walking out of frame as he entered the upmarket Millennium Hotel in Grosvenor Square in the heart of Mayfair.
Who killed him and why? An intelligence source told the BBC he was murdered on the orders of the Russian state because he'd crossed two "red lines" by kensoha direct allegations against President Vladimir Putin. The Kremlin has denied any involvement in the crime.
A public inquiry into the murder has been under way for five months. It's about to go into closed session to hear secret intelligence. A BBC team has had unparalleled access to key witnesses and has spoken to confidential sources close to the case. Alexander Litvinenko was taken ill just hours after his meeting at the Millennium Hotel's Pine Bar with Andrei Lugovoi and Dmitry Kovtun, the two former Russian spies he counted as business contacts, even friends.
He was admitted to his local hospital in north London on 3 November, vomiting and in great pain. He told doctors he thought he had been poisoned. At first, local Metropolitan Police officers were involved. But soon word reached the then head of the Met's counter-terrorism command, Peter Clarke. He was saying that he was a former member of a Russian intelligence agency and that he believed he'd been poisoned by some of his former colleagues. He was showing some s of radiation poisoning.
He was losing his hair. But when doctors passed a Geiger counter over his body the readings were negative. He was clearly severely ill, but no-one could work out why. Two weeks after being admitted to hospital, Litvinenko was transferred by ambulance - with a police escort - to University College Hospital in central London for intensive, supportive care. His white cell blood count was catastrophically low. It looked as if his immune system was being destroyed.
He was showing s of bone marrow failure. He was lined up for a transplant but he continued to get worse. His wife, Marina says: "When I came to see him it was exactly what you saw in the picture in the newspaper. After that he never came out from this bed. He was very, very weak already. He tried as hard as possible to give information to police. They were working closely with the police but there were other mysterious people too. A lot of people wearing suits. The police officers were there but there were some people who probably weren't police officers but were possibly members of the security services", Goldstone says.
The medics were told about some bizarre-sounding poisoning cases from the intelligence archives, allegedly involving the KGB. One story seemed to come straight from the s of a spy thriller - an assassination target was killed by radiation after part of his desk was secretly replaced with a radioactive source.
It was all very James Bond. Prof Amit Nathwani, a blood consultant, was a key member of Alexander Litvinenko's medical team. It started with his liver, and then was followed very rapidly by his kidneys and then his heart. We were in a race, trying to work out the cause before some of the other organs were picked-off.
After 18 days in hospital, his condition was still a mystery. As a last resort, it was decided to send small blood and urine samples to Britain's top-secret nuclear research site at Aldermaston in Berkshire. Scientists at Aldermaston are more accustomed to working with nuclear weapons, but they used their expertise to search for radioactive poison. They jenosha used a technique called gamma spectroscopy.
This advanced analytical technique involves passing energy through the sample in a vacuum to search for radioactive elements emitting gamma rays. Each element has a unique al at a particular energy level. The looked negative. However, they noticed a small spike in the read-out, barely above background levels, at an energy of Kilo electron volts KeV. By pure chance another Aldermaston scientist, who'd worked on Britain's early atomic bomb programme decades ago, happened to overhear his colleagues discussing this small spike in the trace.
He recognised it immediately as the small kehosha ray al from polonium He knew this because polonium was a vital component of early nuclear bombs. Suddenly, everything made sense. It explained the failure to detect radiation poisoning in hospital. Polonium emits vast amounts of energy as alpha radiation but it hardly emits any gamma radiation at all.
This is why the hospital tests with Geiger counters were negative. One of the world's most eminent particle physicists, Prof Ian Shipsey, who was part of the team that discovered the Higgs-boson particle, explains that polonium is a very strong alpha particle emitter.
It produces a lot of energy but in a confined space because alpha particles are easily blocked, for alaja by paper or skin. Hence it is harder to detect. If it's ingested in the body it destroys cells. Alexander Litvinenko had drunk contaminated tea at his meeting at the Millennium Hotel.
He was being killed from the inside. On that Wednesday night, 22 November, doctors at UCH were informed that the poison was probably polonium Not that there was much they could do. Once Litvinenko had drunk the contaminated tea kenoshw 1 November, there was no going back. This was a death sentence. The implications for public health were profound. This was in effect a radiological attack on the capital. The government's Radiation Protection Division, based in Chilton, Oxfordshire, assembled a crisis team of 20 scientists.
They worked through the night. How should they test for contamination? What about the doctors, nurses, his family, his house? And how about the hundreds of people he'd been in contact with over the past three weeks? A crisis was unfolding at alarming speed. Back at Aldermaston, work was continuing to confirm the polonium findings. Next, they tested kenowha larger urine sample with a special kind of spectroscopy deed specifically to detect alpha radiation.
By the morning of Thursday 23 November they had their result: Confirmed - polonium His wife Marina was allowed to say goodbye. For the police, this had just turned into a murder investigation.
At its height, the Met had more than detectives on the case. If Litvinenko had died a week earlier, it would have been an "unexplained death". As it was, there was a trail of radioactivity to follow across kenosa capital and beyond. When the polonium was discovered, Marina Litvinenko was told it was not safe to stay at her home.
I had only minutes to get some things with me, and be out of the house. The government's civil contingencies committee, Cobra, met four times in the week after the attack. They were concerned about causing alarm by closing contaminated hotels. A source told Newsnight that they even tested the London Underground zlana both trains and stations - and found traces of polonium.
This remained secret at the time to avoid public panic. And of course the public were very understandably very concerned," says Peter Clarke. More than 40 sites were contaminated. The sushi bar where they had lunch was kenisha. This is thought to be the day of a escodt murder attempt. Heavy contamination was later found in both their rooms. Three days later he flew from Moscow to London.
Polonium was found on his British Airways flight. And Kovtun flew from Moscow to Hamburg on 1 November. Again polonium was found at locations he visited in the city. This was the day when both Kovtun and Lugovoi met Litvinenko in the Pine Bar at the Millennium Hotel - the place with some of the most heavily contaminated locations of all.
Hotel security cameras captured Lugovoi, and then Kovtun, visiting the bathroom opposite the Business Centre alqna Litvinenko arrived. Lugovoi has his hand deeply alxna in his pocket.
Was he carrying the poison?