Here is one commenter’s post as to why she thinks the police may want to ‘protect’ Hannah and cover this up:
1. They jumped to conclusion too soon at the beginning before doing thorough investigation, and was very positive about ‘Hanna was only a victim’. If the story turned out to be different. They won’t look too good.
2. They sent Amber alert to everybody’s phone in that area which interrupted people and already upset some (Because people were in the middle of conference calls, trading, video games, etc.). If Hannah was involved or even was the killer, then the police have to admit the Amber alert was a big mistake as well.
3. It would be more difficult for them to justify the killing of DiMaggio.
Brett Anderson at 20 minute mark:
Brett states that she missed the cheerleading camp, and that she had to have known mom and bro were dead, and that there was nothing hinky going on between them.