THE terrorist explosions that ripped through Madrid’s crowded commuter trains on the morning of 11 March 2004 killed 191 people, wounded some 2,000 more and prompted an international manhunt for the perpetrators. Soon after, Spanish investigators searching the area near one of the blasts discovered an abandoned set of detonator caps inside a plastic bag that bore a single, incomplete fingerprint. They immediately shared the clue with law-enforcement colleagues around the world. And on 6 May 2004, the US Federal Bureau of Investigation (FBI) arrested Oregon lawyer Brandon Mayfield, proclaiming that his print was a match…

This article first appeared in Nature on 17 March 2010. To continue reading click here.