Avoiding Suspicion
Josef managed to avoid suspicion about Elisabeth's whereabouts from the concerned public by continuously making her write letters to her family. He would dictate what to say, with messages about how she couldn't cope raising her children and wouldn't be coming home for a long time.
He made sure to cover his tracks by driving miles and hours away just to mail the letters with the correct postage stamps to Rosemarie Fritzl. Josef really thought of everything. No one had had any direct contact with Elisabeth for over a decade but also never had reason to question if these three babies were Rosemarie and Josef's grandkids.
