Deathbed accusations, furious denials, and key questions surrounding spy's death

Who killed him?

The former Russian spy and his supporters have pointed the finger at Vladimir Putin, the Russian President.

They believe Mr Putin's agents carried out the assassination to silence a long-term critic. Alexander Litvinenko's deathbed statement said of Mr Putin: "You have shown yourself to be as barbaric and ruthless as your most hostile critics have claimed." Mr Putin has denied the claim and said last night that there was no proof that Mr Litvinenko's death was "violent".

Any other possible suspects?

As a former spy, maverick, and critic of Mr Putin, Mr Litvinenko is likely to have made a considerable number of enemies - although, if he was murdered, his assassin would have needed access to a radioactive source. One possibility is that a rogue element within the Russian intelligence or criminal underworld is responsible.

How did he die?

He was killed by a massive dose of a radioactive element, according to the Health Protection Agency. A large quantity of alpha radiation from polonium 210 was detected in his urine, apparently just a few hours before his death in University College Hospital on Thursday night.

Has the radioactive element been found anywhere else?

Traces of polonium 210 were found at the Itsu sushi restaurant in Piccadilly, where Mr Litvinenko met the KGB expert Mario Scaramella on 1 November, and at the Millennium Hotel in Grosvenor Square, where he met a Moscow-based businessman and two other men earlier that day. Traces were also found in his home in Muswell Hill, north London.

Could he have killed himself?

There is no obvious reason for such an elaborate suicide and Mr Litvinenko prided himself on his fitness. Having said that the police have yet to rule it out, and traces of the radioactive material have been found at his home. Most of the evidence, however, so far points towards a deliberate poisoning.

So is this a murder inquiry?

No, not yet. It is currently being treated as "unexplained" and not a murder. Inquiries are focused on discovering how the radiation got into his body but if evidence points to foul play, then a murder investigation will take over. There is also the crucial question of where did the material come from?

Who is heading the investigation and why?

A team of anti-terrorist officers from Scotland Yard's counter-terrorism command. The use of these officers reflects the "radioactive" element of the case, plus these elite detectives work on cases where the British intelligence agencies, MI5 and MI6, have an interest. The officer in overall command is Deputy Assistant Commissioner Peter Clarke.

What will the police do next?

Detectives will be trying to recover CCTV footage of the former spy in the hotel and sushi bar to discover whether anyone tried to spike his food or drink. Police will also trace possible witnesses.

What are the diplomatic implications?

The case has the potential to become a major diplomatic incident as well as having serious security implications, if radioactive material is found to have been smuggled into this country. If Russian agents are implicated, it could lead to the frostiest relations with Moscow since the Cold War.