The past history with the GF reporting him to Police for making bombs in his RV shows he WAS a threat, but that the FBI disregarded the report because "we don't have any history on him" which, in perfect hindsight, was a massive mistake on their part and something they should be doing a very strong After Action report on.
The claim was made by a person who was in the process of a mental health crisis so bad that an ambulance was summoned to take them to hospital for a psychiatric evaluation. That wasn't deemed to be enough evidence to get a warrant for forced entry to his property or to arrest him. The police investigated as far as they could without a warrant and asked the FBI and the DoD if they had any indication he was a threat. They didn't. It wasn't as simple as there being proof he was a threat and the FBI ignoring it.