Litvinenko's friend Alex Goldfarb said today that all the roads are leading to Moscow. Is he right?

Family and friends of Alexander Litvinenko say that the discovery of traces of polonium-210 on two British Airways planes serving Moscow is damning proof that the source of the radioactive isotope and the assassin both came from the Russian capital. Investigators cannot say for sure when the first traces of polonium-210 were left on board: on a flight from Moscow to London or a return journey.

Police are trying to check the travel itinerary of a number of Russians and the journeys they have made from October 25th. Government officials in Westminster stress even if the poison plot is found to have originated in Russia this does not automatically implicate the Kremlin.

So what are the main unanswered questions for Scotland Yard?

Mindful of the diplomatic sensitivities of this case, detectives are being extraordinarily cautious. Some in the Yard argue they are being too reticent. Police still won't officially say this is a murder.

Of course, unofficially, officers recognise this is the first case in the world of a man being killed by the acute effects of alpha radiation. The priority for the police is to determine where, when and how the polonium-210 was given to Litvinenko. They are trying to retrace his entire journey from the time he left his Muswell Hill home on the late morning of November 1, to detect the first place he began to emit signs of radioactive poisoning.

So far the earliest location is the Itsu sushi bar in Piccadilly. Police say they also want to identify the source of the isotope before they seriously begin to consider the many, many theories as to who wanted to harm Litvinenko

How could you explain the fact that so many aircraft appear to have been contaminated — and that John Reid says even more might need to be checked?

A number of characters based in Moscow who are being checked as part of this investigation made numerous flights to and from London from late October to the end of November. Police still have no idea how many people were involved in this alleged conspiracy. They are also checking the movements of Russians who admitted to meeting Litvinenko on November 1.

These men would have a plausible explanation for how they carried infinitesimal amounts of the substance on board as they shook hands and embraced Litvinenko during their visit together at the Millenium Hotel.

How much credibility are investigators giving to the Kremlin's line that anti-Putin forces are responsible?

They have not ruled out any group. They are perplexed by the incredible number of allegations that have been made as to the motive for this death. The man implicated by Russian MPs and Kremlin sources in the affair is Boris Berezovsky, the London-based oligarch, Putin critic and friend of Litvinenko.

But he has mourned the death of the former spy, describing him as an ally and a close friend who saved his life. After radiation was found in his offices, Mr Berezovsky said he had complete faith in the police investigation. What is emerging is that Litvinenko had a formidable list of friends and foes.

How is the alleged poisoning of Yegor Gaidar, the former Russian PM, tied in to the Litvinenko case?

Police say they are "interested" in developments. There may well be no link. One of the men that Litvinenko met on November 1 used to be the personal bodyguard of the Prime Minister during his days in the Kremlin.

Are the Russian authorities co-operating with the inquiry as they have promised?

Thus far they are offering every technical assistance possible, particularly in relation to the examination of planes and the passenger manifest lists. The Public Prosecutor's office in Moscow has gone on record to say that Yard detectives are welcome to visit the city and question anyone they want. That spirit of co-operation has yet to be tested.

Can we expect to learn more from tomorrow's post-mortem?

The experts hope that the post mortem examination might reveal what other elements were ingested by Litvinenko. They are looking for thalium and bismuth. The hope is that they may be able to say for sure whether this poison originated from a reactor or some other nuclear installation. The findings may be used in any future criminal enquiry.

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