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Richard Barrett, a former head of counter-terrorism at MI6 tells the World at One that a "sophisticated amount" of cooperation between security services is required to enable them to manage the volume of potential suspects travelling between countries. As the identities of the perpetrators of the latest attack on the UK are revealed, a common phrase emerges in the newspapers - that they were "known to security services". Mr Barrett says the difficulty is the volume of people and also the speed at which someone moves from being radicalised to taking action, which he says has changed from "years to possibly a matter of months or weeks." (Photo: 3 suspects of terrorist attack on London Bridge Credit: Getty Images)