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***, as in this translation:

> Die Polizei teilte am Donnerstag mit, sie könne noch nicht abschließend feststellen, was Kamphuis genau zugestoßen sei. Derzeit ermittle sie in drei Richtungen: "freiwilliger Abschied" einschließlich eines möglichen Suizids; ein Unfall; oder ein Verbrechen.

Rationale: 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. 

Side issue:

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 is still alive but does not want to be located by authorities. "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.