My character was kidnapped by a serial killer two and a half months before my story begins (he let her go because he only kills people who have done something he deems 'bad' and then sets up their deaths to look like a suicide/mugging gone wrong to 'punish' them, in his mind she had done nothing wrong), at the end of the story she will come face to face with the guy when he kills her mother-in-law (who is also his biological mother who abandoned him, thus making him into the man he became.

Through the story my MC keeps finding deaths which may actually by murders carried out by the killer. My problem, she knows the killer's name but there isn't enough evidence to convict him of the murders. But she knows who he is. Why can't they just go pick him up if they know his name? I'm wanting him to crop up throughout the story without getting caught until the end. Any suggestions?

Also, what could the killer do to crop up in the story? I'm wanting the MC to be worried about telling people that she thinks she's seeing him in case they think she's being paranoid. So far I've got the idea of seeing him in a crowded supermarket, on public transport, when she's out investigating another case.

They're in Scotland if that makes any difference. Thanks in advance for any suggestions.