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The number of murders in the US rose nearly 30 percent last year, the vast majority of them committed with guns. One company called ShotSpotter believes its technology can help tackle America's growing gun crime problem by alerting police the moment shots are fired, but it has become increasingly controversial. James Clayton goes behind the scenes at ShotSpotter and on patrol with Fresno police to see how it works - and meets the campaigners in Chicago who say the technology cannot be trusted.