London-based Olive Security provides former British Special Air Service troops--one unarmed guard for every four or so engineers who go out on a job--to drive and offer a show of muscle.

FORBES: On The Cover/Top Stories

His assassins said by British officials to have had help from Russia's security service nearly got away with it.

ECONOMIST: Assassinations and technology

The diaries of former deputy MI5 chief Guy Liddell provide a unique insight into the Security Service during some of the darkest days for British intelligence.

BBC: Cambridge Spies: MI5 diaries revealed

Last month, British officials announced they had enough evidence to charge Lugovoi -- a former security service agent-turned businessman -- with Litvinenko's murder.

CNN: UK slams Russia on poison killing

Litvinenko's friends are urging British officers to question a jailed Russian intelligence officer who has accused the Russian Federal Security Service, the FSB, of creating a hit squad to silence Russian emigres in Britain.

NPR: U.K. Investigators Visit Moscow in Litvinenko Probe

British police say they have amassed detailed evidence against Mr. Lugovoi, a career security service officer, and tracked a trail of polonium across Europe to his London hotel room and the upscale Mayfair bar where he met with Mr. Litvenenko when he consumed some polonium-laced tea.

WSJ: Russia Declares Litvinenko Murder Suspect a Victim