Obituary: Alexander Litvinenko

Alexander Litvinenko, who has died in a London hospital aged 43 from a mysterious illness, was a career security services officer who got sucked into the dark underworld of Russian politics.

Born in Voronezh, south-west Russia, he joined the army out of school, rising to the rank of lieutenant colonel. Then, in the dying days of the Soviet Union in 1988, he entered the counter-intelligence department of the KGB.

In 1991, once the KGB's directorates had split up, he worked for the federal security service (FSB), fighting terrorism and organised crime, sometimes operating in Chechnya. In 1997 he moved to one of the most secret divisions of the service, a unit called URPO investigating "organised criminal formations".

At this time, a former mathematician turned businessman called Boris Berezovsky was making his fortune car dealing and picking up chunks of state companies in dubious privatisation deals. Shrewd, manipulative and charged with boundless energy, Berezovsky soon inveigled his way into the Kremlin, becoming a power behind the throne in the later years of the Yeltsin presidency.

As Yeltsin's health and popularity waned, Berezovsky, one of the first tycoons to be called an oligarch because of his political heft, needed allies to protect his position as rivals closed in on his business holdings. He fell in with Litvinenko, who had earlier investigated a car bomb attack in which Berezovsky narrowly escaped.

Then, in 1998, Litvinenko called a press conference, claiming that a year earlier he had been instructed to kill Berezovsky by then deputy head of the Russian security council. Flanked by other members of his FSB unit, one in a black balaclava, Litvinenko said his superiors had threatened him with violence when he refused their order to "kill the Jew who'd robbed half the country".

The truth of the accusation would remain disputed. Critics said it was fabricated to help Berezovsky blacken enemies in the FSB. Litvinenko claimed it was just one manifestation of the corruption and violence inside the FSB that he wanted to expose.

He was arrested the following March and imprisoned in the FSB prison at Lefortovo in Moscow on charges of exceeding his authority at work. He was acquitted in November 1999 but re-arrested before the charges were again dismissed in 2000. A third criminal case began but Litvinenko secretly left the country, ending up in London with his wife Marina, and was granted political asylum. In 2002 he was convicted in his absence in Russia and handed a three and a half year jail sentence.

In Britain, Litvinenko was reunited with Berezovsky, now too living in self-imposed exile. Litvinenko wrote two books, the first, The FSB Blows up Russia (2001), implicated the security services in a series of apartment block bombings in 1999 that killed more than 300 people. The attacks were blamed on Chechen rebels, but his book echoed fears of state involvement as a means of justifying the second war in Chechnya. The other was The Criminal Group from the Lubyanka (2002). However, other accusations, such as that FSB agents trained al-Qaida operatives in Dagestan and were involved in the September 11 attacks, did little for his credibility.

Before his death he was said to be investigating last month's assassination in Moscow of his acquaintance, the investigative reporter Anna Politkovskaya (obituary, October 9).

What is certain is that Litvinenko will remain a disputed figure; to some a courageous defector and whistleblower, to others a traitor and oligarch's sidekick.

He is survived by Marina and their son, Anatoli.

· Alexander Valterovich Litvinenko, security service officer, born 1962; died November 23 2006