Again and again, the standard DNA tests came up negative on a three-inch-by-two-inch piece of muscle recovered from the World Trade Center site, and just a year after the 2001 attacks, forensics experts were stymied. Yet now, the scrap has been linked to a firefighter from Midtown Manhattan, allowing his death to be confirmed and giving his wife and two children some sense of finality. Solving brutally difficult cases like that one required an investment of two extra years and millions of dollars by the medical examiner's office in New York, which sought out and used DNA identification technologies that...