Detectives investigating the death of Alexander Litvinenko were last night examining the possibility that the former spy killed himself to discredit Vladimir Putin.
Increasing concerns over the reliability of the Russian dissident's death-bed testimony have prompted police to check every detail of Mr Litvinenko's version of events on 1 November, the day he said he was poisoned.
The Russian dissident's death on British soil has triggered an unprecedented investigation headed by Scotland Yard's anti-terror branch and involving forensic experts and nuclear scientists from the Atomic Weapons Establishment at Aldermaston. They are still trawling through hours of CCTV footage and conducting detailed searches of the places he visited on the day he fell ill.
Meanwhile, nuclear scientists are frantically trying to establish just how radioactive was the dose of polonium-210 that killed Mr Litvinenko. Traces of the material - powerful enough to trigger a nuclear warhead - were found on tables at the Itsu sushi restaurant in Piccadilly, a London hotel, and his home in Muswell Hill.
But yesterday the Metropolitan Police were still treating Mr Litvinenko's death as an "unexplained death", not as a murder inquiry. One source close to the investigation said: "He was a guy with a colourful past. It's not straightforward."
Officers are working on several theories, including the seemingly implausible possibility that he took his own life.
The 43-year-old's death on Thursday evening has led to a health scare, with officials yesterday urging anyone who came into contact with Mr Litvinenko to contact a special helpline. The Health Protection Agency has stressed that the risk is minimal, but has also admitted that this is an "unprecedented" incident.
Detectives are still no nearer to establishing just how Mr Litvinenko, a fierce critic of President Alexander Putin's regime, came to ingest such high doses of polonium.
The presence of radioactive particles in the restaurant where he ate more than three weeks ago adds weight to theories that the poison could have been sprinkled over his food.
Some reports in the Russian press have suggested that Mr Litvinenko's death could have been a "martyrdom operation", on the grounds that no state would want to attract the attention of a radioactive poison plot.
But British officials warned against assuming that the spy staged his own dramatic demise.
One senior source warned: "You have to remember this guy was on his guard 24 hours a day. Normal assassination methods may well not have worked."
Additional reporting Sonia ElksReuse content