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The F.B.I. believes that less than one per cent of the killings each year are carried out by serial killers, but Hargrove thinks that the percentage is higher, and that there are probably around two thousand serial killers at large in the U.S. “How do I know?” he said. “A few years ago, I got some people at the F.B.I. to run the question of how many murders in their records are unsolved but have been linked through DNA.” The answer was about fourteen hundred, slightly more than two per cent of the murders in the files they consulted. “Those are just the cases they were able to lock down with DNA,” Hargrove said. “And killers don’t always leave DNA—it’s a gift when you get it. So two per cent is a floor, not a ceiling.”