Given your (subsequently added) context
Police said on Thursday they were "holding all possibilities open in respect to what might have happened" to Kamphuis and pursuing three distinct lines of inquiry: a "voluntary disappearance" including a possible suicide; an accident; or foul play.
I suggest using for foul play simply ein Verbrechen
Die Polizei teilte am Donnerstag mit, sie könne sich noch nicht festlegen, was Kamphuis genau zugestoßen sei. Derzeit ermittle sie in drei Richtungen: "freiwilliger Abschied" einschließlich eines möglichen Suizids; ein Unfall; ein Verbrechen.
In a relatively sober, factual report like this I do not see an option to use for "foul play" some of the German equivalents like "ein krummes Ding", "eine faule Sache" etc. as these are expression applicable only in very informal speach - rather in the pub, even not at the workplace.
The bigger problem here in my perspective is how to treat the "voluntary disappearance". Directly translated "freiwilliges Verschwinden" it sounds unprofessional and would not be part of a statement by the police. Usually they would say "freiwilliges Untertauchen", but this means that the person wants to be not found by authorities but is still alive. "Freiwilliges Untertauchen einschließlich ein möglicher Suizid" are contracting therefore and could be part of such a police statement only if written by an unexperienced junior officer (which sometimes happens, but most regularly they pass statements through a press liaison bureau that irons out stylistic mishaps). Therefore, eventually, I decided to use "freiwilliger Abschied" in this translation as this can include death. It is very unusual, though.