There are an estimated 250 000 unsolved murders in the US, and in many of them, the victims are yet to be identified. But now, recent breakthroughs in DNA technology and the growth of genealogy websites are beginning to unlock the secrets of cold case murder investigations. James Clayton travels to Missouri to discover how cutting-edge science has finally identified a woman killed 30 years ago and who was simply known as Grace Doe for decades. The technique has brought her family a form of closure, and it looks set to solve thousands more cases. But does it also come at a cost to our privacy?