foul play - ... 2: unlawful or dishonest behavior, in particular violent crime resulting in another's death.
foul - 1: ... e: ...
the police do not suspect foul play - die Polizei vermutet kein Verbrechen
foul - 1: ... d: (= dishonest, unfair) ...
the police do not suspect foul play - die Polizei hat keinen Verdacht auf einen unnatürlichen /or/ gewaltsamen Tod;
there was no evidence of foul play - es gab keine Beweise, die auf Mord /or/ einen gewaltsamen Tod schließen ließen
I have to agree with Carullus, Caravan, Ronnie, and Juskys (hopefully not all the same person) on this one. 'Foul play' does not simply mean murder; that 1:1 translation would just be wrong. It's a deliberately vague explanation of how something bad happened to someone, implying that something illegal was involved, but precisely without jumping to conclusions about what that was.
The 'something bad' could possibly have been murder, but possibly something else. If someone has disappeared, it may be impossible to say whether it's murder or kidnapping or Alzheimer's or running away or what. Even if someone has been found dead, the death could have been caused by an accident or an illness, or negligent homicide or manslaughter or some other crime short of murder. Some of those are foul play, others are natural causes; only if something is in the general, broad category of 'foul play' are the police required to investigate further, in order to determine whether the suspicion of foul play is correct and if so, what kind of foul play (and therefore what kind of crime) it was.
The key phrase in the definition is 'resulting in another's death.' NOAD is more precise here than M-W, and therefore less open to misreading.
Expressions like this are used precisely during the stages before the coroner issues a verdict and the district attorney decides whether to charge anyone with a crime, and if so, which crime. So the vagueness is deliberate, and the translation should not remove it.
Is it not possible to use something equally vague, like 'kriminelles Verhalten' or 'unnatürliche/gewaltsame Umstände'? How about 'verbrechenverdächtig'? Okay, I know that probably doesn't exist, but isn't there anything that comes close? It just seems to me that there must be any number of ways to say this without being as misleading as using the word Mord.
Would it help to include a sentence with 'suspect foul play,' which is such a frequent collocation?