apparently it is because you got it wrong. they found the threat to be serious enough to investigate but not credible enough to be actionable.
let me explain the difference.
If i report that doug has threatened to murder me, well murder is a serious crime and people who threaten it sometimes end up murdering those they threaten.
so the police found the threat serious enough to investigate but when they found that doug is my neighbor's chicken, the threat of murder wasn't credible.
because the threat wasn't credible they did not pursue prosecution.
hope that helps show the difference between brianna crying wolf and that being serious enough for the FBI to investigate and her being batshit crazy and/or full of shit and justly the FBI deeming her maniacal rantings not credible.