It was rather inexplicable, but I supposed that, for some reason or other, Japp wished to keep it out of the papers. 
Japp was probably laying some sort of trap for catching the murderer. 
mnli hypothesis: Japp was probably laying some sort of trap for catching the murderer.  premise: It was rather inexplicable, but I supposed that, for some reason or other, Japp wished to keep it out of the papers. 