British anti-terrorist detectives are poised to fly to Russia and Italy in an effort to solve the fatal poisoning of the Russian dissident Alexander Litvinenko.
But as John Reid, the home secretary, said the police inquiry had been upgraded from an "unexplained" to a "suspicious" death, experts voiced doubt at the theory that anyone acting alone could have possibly used the isotope polonium 210 to kill Litvinenko.
One scientist said polonium 210 would only kill so quickly if combined into a "designer toxin" with another isotope, beryllium, in a complicated process that would require state sponsorship. Such a process was used by Britain in early atomic weapons in the 1950s.
"No individual could do this," said John Large, an independent nuclear consultant.
"What you are talking about is the creation of a very clever little device, a designer poison pill, possibly created by nanotechnology. Without nanotechnology you would be talking about a fairly big pill, a pea-sized pill. Either way you are looking at intricate technology which is beyond the means and designs of a hired assassin without a state sponsor," he said.
He said the likely poison pill that killed Litvinenko would have to have been manufactured in a special laboratory over two or three weeks and then used very quickly -- possibly within 28 days -- because the half-life of the isotope polonium is only 138 days.
Senior police officers are drawing in experts from the International Atomic Energy Authority, and from the Atomic Weapons establishment at Aldermaston in the UK.
Every option is being considered, from Kremlin involvement to the theory that Litvinenko's work in the anti-corruption unit of the FSB, Russia's FBI, created enemies with the means and knowledge to assassinate him.
The government's emergency planning group, COBRA, has met at least six times in the last few days. Police are studying hours of CCTV tapes to trace Litvinenko's movements on and before Nov. 1, when it is likely he received the dose of radiation that killed him.
Officers may travel to Russia to interview Andrei Lugovoi and Dimitri Kovtoun, who met Litvinenko in the Millennium Hotel, London, on Nov. 1, and to Italy to speak to Mario Scaramella, who met him at the Itsu restaurant in Piccadilly, London.
"If we need to go to Russia and Italy we will do that at the appropriate time," one police source said yesterday.
Kovtoun, who denies involvement, said he was going for radiation tests, because of concerns over his own contamination. One man police may speak to is a former KGB general with links to the Dignity and Honor group of retired KGB officers.
The man is named in a document passed by Scaramella to Litvinenko as the ringleader of a group which could be planning to kill both men. He is understood to have left Moscow on Friday for an unknown destination.
The home secretary refused to be drawn on the police investigation yesterday.
Litvinenko's home in north London, where traces of alpha radiation from polonium 210 have been found, was still being examined on Sunday.