He hadn’t lived at home for years.
As far as knowing, psychopaths are generally above average intelligence-many are geniuses. They manipulate people instinctively and use the inherent trust of others without any remorse. Unless someone is proven to be collaborating, they are just another victim. That doesn’t even address the fact that many warnings are disregarded by professionals—see the last school bomber.
From what I have seen the ATF, FBI and local police have gutted Conditt’s house. The house is practically destroyed.
They have done their job.