For more than a century after Jack the Ripper became one of history's most feared and famous murderers, his identity remained a mystery. Then last year, on a tour of Scotland Yard, a best-selling American crime novelist met a leading British authority on the Ripper. In an excerpt from the resulting book, PATRICIA CORNWELL reveals how she and other investigators used state-of-the-art forensic science, including DNA tests, to make the case that the serial killer who terrorized Victorian England was the handsome, gifted young artist named Walter Sickert.