I searched for some example sentences, to make sure I get the conotation and use of *foul play* in the English language correct (because I'm no English native speaker) and found 
> She is still missing, and the police now suspect that she may have been a victim of foul play

and

>There is no evidence of foul play

 at the [Learner's Dictionary][1]. This sounds like the expression is also used in official news in e.g. a newspaper and not only in colloquial language.

In the given context one of the following German expressions is usually used:

> Verbrechen, Straftat, Delikt, Gewalttat


The translation for the first example sentence could be:

> Sie wird immer noch vermisst und die Polizei vermutet nun, dass sie Opfer eines Verbrechens geworden sein könnte.

and the second example sentence in German:

> Es gibt keine Anzeichen einer Straftat.

 [1]: http://www.learnersdictionary.com/definition/foul%20play