Someone killed at least four children in Oakland County, Mich. between 1976 and 1977.
Investigators have been working for decades to determine who the serial killer is and if he is still alive. Those who knew the victims -- their families, friends, classmates, teachers, coaches and neighbors -- have been forever changed by what happened during the 13-month period from February 1976 to March 1977.
In that time, the four children were abducted and murdered with their bodies left in various locations within or just outside Oakland County.
One theory around the unsolved case revolves around North Fox Island, a remote island in Northern Michigan tied to a child pornography ring.
Frank Sheldon, originally from Southeast Michigan, owned the island. He was from a wealthy family, and by all accounts, was a nice and friendly guy.
In the late 1970s, Sheldon was charged with criminal sexual conduct in connection to what police said was a child porn ring, which was operating through North Fox Island.
It wasn't until years after that a connection to the unsolved Oakland County Child Killer case starting to emerge.
Watch Chapter One of "Child Killer: North Fox Island" in the video player above.