The following points are key developments in the poisoning of former KGB agent Alexander Litvinenko:

Nov. 1 — Litvinenko meets at a London hotel with another former KGB agent, Andrei Lugovoi, along with Dmitry Kovtun and Vyacheslav Sokolenko. Afterward, he goes to a sushi bar to meet Italian academic Mario Scaramella. He later falls ill and is taken to a London hospital.

Nov. 20 — Litvinenko is moved to intensive care and photos of him looking gaunt in his hospital bed are released. Police say their counterterrorism unit is investigating.

Nov. 23 — Litvinenko dies.

Nov. 24 — A statement dictated by Litvinenko is released blaming Russian President Vladimir Putin for his death; Putin's government denies involvement. Britain's Health Protection Agency, the HPA, says the rare radioactive element polonium-210 had been found in Litvinenko's urine. The agency describes the poisoning as "unprecedented" and the government convenes a crisis committee. Police say traces of radiation have been found at a number of sites, including Litvinenko's north London house, the sushi bar and the hotel.

Nov. 29 — HPA says it will test more than three dozen staff at hospitals that treated Litvinenko for radioactive contamination.

Nov. 30 — Authorities find small traces of radiation aboard two British Airways jetliners.

Dec. 1 — Scaramella tests positive for a significant quantity of polonium-210.

Dec. 4 — Scotland Yard detectives fly to Moscow to interview a number of potential witnesses. Russian Foreign Minister Sergey Lavrov warns persistent suggestions of Russian involvement in the death could hurt relations between London and Moscow.

Dec. 5 — Russia's chief prosecutor Yuri Chaika says no suspects will be extradited to Britain and all Russian citizens suspected of involvement would be tried in Russia.

Dec. 6 — Scotland Yard announces it is treating the death as murder.

Dec. 7 — Litvinenko is buried in north London cemetery. Seven Millennium Hotel employees test positive for polonium-210.

Dec. 9 — German police say they have found they found traces of radiation at two sites in and near Hamburg linked to Kovtun.

Dec. 11 — Putin spokesman Dmitri Peskov insists it is "unthinkable" the Russian government could be behind any killing.

Jan. 27 — Lugovoi criticizes media reports naming him as a suspect and denies any role in the killing.

Jan. 31 — Scotland Yard says it has sent a file on the case to prosecutors.

March 29 — Scotland Yard meets Russian detectives investigating Litvinenko's death in London.

April 3 — Litvinenko's widow, Marina, launches the Litvinenko Justice Foundation to campaign for his killers to be brought to justice. HPA says 17 people have tested positive for polonium-210 contamination at a level which may produce some small long-term health risks. Several hundred people showed minor traces of the element.

May 22 — British prosecutors request the extradition of Lugovoi so he can be charged with Litvinenko's murder.