People are citing various definitions or examples, but the one that is the most important is the definition of 'domestic terrorism' contained in the federal statute 18 US Code s/s 2331:
(5)the term “domestic terrorism” means activities that—
(A) involve acts dangerous to human life that are a violation of the criminal laws of the United States or of any State;
(B) appear to be intended—
(i) to intimidate or coerce a civilian population;
(ii) to influence the policy of a government by intimidation or coercion; or
(iii )to affect the conduct of a government by mass destruction, assassination, or kidnapping; and
(C) occur primarily within the territorial jurisdiction of the United States;
Meet those conditions and the FBI/DOJ would have grounds to invoke a federal mandate. Even if invokes, in any prosecution, the above conditions would have to be proven.