“So the FBI knows what the motive is and is stalling until this hot story cools.”
Exactly. If the truth came out a few hours after it happened, they know most Americans would demand actions against them (whether ISIS, Antifa, or ??).
The FBI is just dribbling out the information for less effect.
Austin City Limits immediately offering refunds tells me a larger plot was eiscovered, not a one-off lone wolf.