COVER Bullets fired from the same pistol by the Prince George's County Police Department, in Maryland, show similar but not identical markings—an example of the ambiguity that plagues much crime-scene evidence. The bullets were fired as part of a research effort to bring greater rigor to forensic analysis, conducted by the National Institute of Standards and Technology. See page 1128. For more on the process behind the cover image, see http://scim.ag/1LPKy4C.

Photo: Matthew Rakola