ALEXANDER LITVINENKO
Alexander Litvinenko became an officer in the FSB's predecessor, the Soviet-era KGB, after transferring from the military.
He was reported to have fallen out with President Vladimir Putin, then head of the FSB, in the late 1990s, arguing that the organisation had not done enough to tackle internal corruption.
He accused FSB agents of plotting to assassinate the then powerful tycoon Boris Berezovsky, who himself now lives in self-imposed exile in the UK.
Mr Litvinenko later wrote a book alleging that FSB agents co-ordinated the 1999 apartment block bombings in Moscow that killed more than 300 people. The Russian government blamed Chechen separatists for the blasts and later that year Russia poured troops into Chechnya in a new offensive.
Mr Litvinenko was thought to have been close to journalist Anna Politkovskaya - a fierce critic of the Kremlin's policies in Chechnya. Politkovskaya was shot dead in October in Moscow, and Mr Litvinenko had said he was investigating her murder.
Mr Litvinenko died in London in November.
ANDREI LUGOVOI
Andrei Lugovoi is a former KGB officer who now heads a private security firm.
The UK's director of public prosecutions has said that he should stand trial for Mr Litvinenko's murder.
He met Mr Litvinenko on 1 November at London's Millennium Mayfair Hotel - the day Mr Litvinenko fell ill.
Mr Lugovoi denies any wrongdoing, suggesting that someone has been trying to frame him.
He underwent medical checks in hospital after traces of polonium-210 were reportedly found in his body.
He was questioned by Russian and British detectives in December as a witness.
Mr Lugovoi says he became acquainted with Mr Litvinenko 10 years ago, but it was neither a friendship nor business partnership.
Mr Lugovoi says his security work brought him into contact with Boris Berezovsky, who also knew Mr Litvinenko.
DMITRY KOVTUN
Dmitry Kovtun is another former KGB officer-turned-businessman, who met Mr Litvinenko at the Millennium Mayfair Hotel on 1 November.
Mr Kovtun - a business associate of Mr Lugovoi - denies any involvement in Mr Litvinenko's death.
Mr Kovtun - who has also been questioned by the detectives - reportedly tested positive for radioactive poisoning and underwent checks in a Moscow hospital.
A car, understood to be used by Mr Kovtun, has tested positive for polonium-210, according German authorities. They have also said that Mr Kovtun's ex-wife, her partner and two children have been contaminated with the radioactive substance.
Mr Kovtun stayed at his ex-wife's flat in Hamburg before flying to London, according to German authorities.
In January, Mr Kovtun and Mr Lugovoi told a Russian television station that they were witnesses, not suspects, in the affair.
Mr Kovtun studied at the same elite Moscow military academy as Mr Lugovoi. The pair were childhood friends who lived in the same apartment block and their fathers also served together at the defence ministry.
VYACHESLAV SOKOLENKO
Vyacheslav Sokolenko - an ex-KGB officer who currently heads a security firm - denies claims that he was the "third man" at the Millennium Mayfair Hotel meeting on 1 November.
However, he admits that he was staying at the hotel at the time.
Mr Sokolenko says he had travelled to London with Mr Lugovoi and his family purely to see a football match between CSKA Moscow and Arsenal.
MARIO SCARAMELLA
Mario Scaramella - an expert on post-World War II KGB spying in Italy - met Mr Litvinenko on 1 November at a sushi bar in central London.
They had met to discuss e-mails claiming that both he and Mr Litvinenko were "under the special attention of hostile people", Mr Scaramella said in an interview with CNN.
He was given a clean bill of health in December, several days after he was hospitalised in Britain by doctors who had found traces of polonium-210 in his body.
THE POLONIUM TRAIL
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