Deep inside a sprawling complex tucked in the hills of this Appalachian town, a room full of supercomputers attempts to sift America's guilty from its innocent. This is where the FBI keeps its vast database of fingerprints, allowing examiners to conduct criminal checks from computer screens in less than 30 minutes--something that previously took them weeks as they rummaged through 2,100 file cabinets stuffed with inked print cards. But the same digital technology that has allowed the FBI to speed such checks so dramatically over the last few years has created the risk of accusing people who are innocent, the...