DNA can be tested instantly
What is Rapid DNA?
My thoughts on the return FBI visit is that the sample they collected a couple of days ago was either inadequate for a DNA test or had been rendered inadequate by cleaning or some other methodology.
That's all that makes sense to me.
Also at the outset, on receipt of the missing Brian report from parents, LE collected several items of his clothing which I assume were for search purposes for the dogs to track.
If those garments contained his scent- sweat- they would also have contained his DNA. From his sweat glands.
Who was it that gave LE the dog's hairbrush? (I know the answer to that one)