This is how it works, at least in civil law countries. If the prosecutor has reasonable suspicious that a crime is taking place they send the so-called "judiciary police" to gather evidence. If they find none (or they're inconclusive etc...) the charges are dropped, otherwise they ask the court to go to trial.

On some occasions I take on judiciary police duties for animal welfare. Just last week I participated in a raid. We were not there to arrest anyone, just to gather evidence so the prosecutor could decide whether to press charges and go to trial.