When an Oregon college student, Mohamed Osman Mohamud, thought of using a car bomb to attack a festive Christmas-tree lighting ceremony in Portland, the F.B.I. provided a van loaded with six 55-gallon drums of “inert material,” harmless blasting caps, a detonator cord and a gallon of diesel fuel to make the van smell flammable. An undercover F.B.I. agent even did the driving, with Mr. Mohamud in the passenger seat. To trigger the bomb the student punched a number into a cellphone and got no boom, only a bust.