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In the wake of the Boston Marathon bombing and the killing of a British soldier on the streets of Woolwich in London, it emerged that the suspects were known to the security services. But how feasible is it for the authorities to keep track of everyone on their watch list?

Michael Adebolajo and Michael Adebowale's names have become well known since they became key suspects in the killing of Drummer Lee Rigby in a street attack in Woolwich on 22 May. But it has transpired that they were already familiar names to the UK鈥檚 domestic intelligence service, MI5. The UK Parliament's Intelligence and Security Committee is to investigate the security service's actions in relation to the case.

Boston bomb suspect Tamerlan Tsarnaev was questioned in 2011 amid claims he had adopted radical Islam. But is it practical - or even possible - to keep close tabs on every person who comes to the attention of the security services?

Tim Harford crunches the numbers with the help of former head of MI5, Dame Stella Rimington; Howard Wainer, distinguished research scientist at the National Board of Medical Examiners; and Professor Louise Amore, a security data analytics expert from Durham University.