I may have missed this but what is the link between AA's house, AA, and ML?
We know something led LE to that site. Was it simply a tip phoned in that he was burning materials and discarding a mattress or was it both that and ML's location history showed she had been there before?
Although they collected a lot of evidence they released him relatively quickly. The linkage must be somewhat circumstantial for them to have let him go.
I have to believe if there was contemporaneous tracking of her electronics to that site that he would still be in custody.