Regarding respectfully BBM -
That kind of killer is most difficult to find - reminds of a woman who worked as secretary in local FBI office, she recently had gotten enough $$ to move into new home, and was killed much like Allison. Because of where she worked, big effort to solve, no DNA matches, etc. ...
Only years later they go a hit on the DNA when the killer's DNA entered the system (I forgot why) and he got caught. Turns out he lived down the street from her, had seen she lived alone and took advantage... It was his first and only time...
The only difference I see here is that Allison was not always alone, and that she there was an alarm sign outside her home...
ETA: I can't remember if he said he had spoken to her, when she first moved in, as if to welcome her to the neighborhood and offer to be of help with anything....
I remember this story. The only problem that I have is would she have opened her door to someone that she didn't know well at night? I just don't see it.