EDIT: Given the new example sentence in the comments of the question, Verbrechen would be the best fit in this specific case.
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. 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.