Law enforcement professionals have a wall that seperates them from their fellow man. A sucessful investigation of a crime is one that results in the arrest and conviction of the guilty.
So that means that the only thing that matters is evidence. Stuff that will convince the judge or jury that this guy did this crime beyond a reasonable doubt. The detectives can speculate all they want, but if they don’t have something to back it up then they leave it outside the door when they walk into a courtroom. If they don’t then they’ll probably lose the case and their credibility will be questioned the next time they come in front of the judge. What’s worse is that the perp will walk and go on to commit more crimes.
Of course, it’s an imperfect world. Sometimes a whole case is built on speculation, and sometimes someone is convicted when they shouldn’t be. We rightly see this as a miscarriage of justice, an instance when our society violates the very principles which make up its foundation. It’s where we all agree that the system breaks down.