The poisoned umbrella 'cover'
13 September 2013 Last updated at 09:36 BST
Bulgaria has closed its investigation into the infamous Cold War murder of dissident Georgi Markov in London after the statute of limitations in the case expired on Thursday, 35 years and one day after his death.
The exiled writer was thought to have been stabbed in the thigh in 1978 by the poisonous tip of an umbrella while he waited for a bus on Waterloo Bridge.
Professor Christopher Andrew, a historian at the University of Cambridge, explained on Today how the murder is believed to have been carried out.
"It wasn't actually an umbrella," he said, "the umbrella was simply a cover for a silenced gun which from beneath the umbrella shot a poisoned pellet into the thigh of Georgi Markov."
First broadcast on the Today programme on Friday 13 September.