To use the FBI's terminology, the national "clearance rate" for homicide today is 64.1 percent. Fifty years ago, it was more than 90 percent.
And that's worse than it sounds, because "clearance" doesn't equal conviction: It's just the term that police use to describe cases that end with an arrest, or in which a culprit is otherwise identified without the possibility of arrest — if the suspect has died, for example.
Criminologists estimate that at least 200,000 murders have gone unsolved since the 1960s, leaving family and friends to wait and wonder.
"It's like the boogeyman," says Delicia Turner. Her husband, Anthony Glover, was found murdered — along with a friend — in Boston in 2009. Police never made an arrest. She says the open case preys on her mind. "You don't know if you're walking next to the person, if you've seen the person ... if the person knows you."