The FBI might have been too busy helping them be better bombers.
In many ways that’s all they can do. Ordinarily they would supply fake explosives, then arrest the perps. I can only guess that Tammy rebuffed their offer, his Chechen and/or Saudi handlers supplied enough, or the feebs didn’t bother attempting that sting. There really is not a lot they can with these terrorists except deport them all.
Well, that would be the ATF’s jurisdiction since they have a lot of experience at this kind of stuff.