Unsolved murders, also known as cold cases, are really starting to pile up across the U.S. That’s because detectives are not clearing homicides like they used to. In 1965, the national homicide clearance rate was 90%. By 2012, that rate had fallen to 64%. As an example, Detroit’s police department arrested only 9% of those responsible for the city’s 386 murders three years ago. New Orleans didn't fare much better, clearing only 15% of its 193 killings in 2012.