Assistant U.S. Attorney Brook Cucinella gave opening statements Monday at the Manhattan trial.
The case stems from the case of a New York City police officer convicted last year on charges he plotted to abduct and eat women.
Prosecutors say family members of Michael Vanhise of Trenton, N.J., were described as targets in Internet chats that Vanhise had with co-defendant Robert Christopher Asch.
Cucinella says the FBI stopped the plots before they could be carried out.
Lawyers for both defendants acknowledged the evidence would be vile. They say it was an exploration of dark fantasies online and the defendants never meant to harm anyone.