Everyone gets tired of the ransacking of private lives, the cynical search for ulterior motives, the weighing of imperfect evidence; after a while, people are likely to say, Just let it go.
Everyone gets tired of violating the privacy of other people and searching for potential ill-intent and trying to decipher incomplete evidence so after a while investigators are likely to just say let it go. 
The investigators have all the information, context and understanding they need to completely make decisions after understanding people's statements and accurately assessing a level of pre-emptive risk. 