It is not only sex-assault but there are many cases of harassment, bullying, stalking that can not be prosecuted. You can report it to the police but without substantial evidence they can not do anything; they will even accuse you of causing the problem. That is the way the law works. 
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People lie, that is the fact. 1 in 100 are psychopaths and other borderline mental cases. Psychopaths can be extremely manipulative and lie well to the police, cops are not superhuman they can be manipulated.
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https://www.youtube.com/watch?v=2NzmKIUCSW8