Smiley face murder theory

The Smiley face murder theory (variations include Smiley face murders, Smiley face killings, Smiley face gang, and others) is a theory believed by retired New York City detectives Kevin Gannon and Anthony Duarte. They say that a number of young men found dead in bodies of water across several Midwestern American states from the late 1990s to the 2010s did not accidentally drown. They believe that they were victims of a serial killer or killers. 
The term "smiley face" became connected to the alleged murders when it was made public that the police had discovered graffiti showing a smiley face near locations where they think the killer dumped the bodies in at least a dozen of the cases. 
Gannon wrote a textbook case study on the subject titled "Case Studies in Drowning Forensics". The response of law enforcement investigators and the Federal Bureau of Investigation to not believe in this theory and had investigated it saying that it is a hoax.