As Americans use digital methods for more of their interpersonal communications, law enforcement agencies have seized the opportunity to scoop up more information for cheaper than they could before, hoping to ferret out criminal activity. But violent crime still takes place in the physical world, with fragile human bodies on the line. A growing number of U.S. police departments are using a system of sound-detecting software to locate and respond to gunfire in hopes of catching more shooters and saving more victims.