Nearly 20 years ago, retired NYPD detectives started tracking the suspicious drownings of young men across the country after they discovered a graffiti smiley face near at least a dozen locations where they found bodies.

Before they're caught, serial killers often fly under-the-radar. But just because police and the public aren't aware of a killing spree, doesn't mean it isn't happening. We look at five unsolved serial killer cases, the murders connected to them and some of the information we do (and don't) know.