THE United States has been narrowly saved from lethal terrorist plots in recent years — or so it has seemed. A would-be suicide bomber was intercepted on his way to the Capitol; a scheme to bomb synagogues and shoot Stinger missiles at military aircraft was developed by men in Newburgh, N.Y.; and a fanciful idea to fly explosive-laden model planes into the Pentagon and the Capitol was hatched in Massachusetts.

But all these dramas were facilitated by the F.B.I., whose undercover agents and informers posed as terrorists offering a dummy missile, fake C-4 explosives, a disarmed suicide vest and rudimentary training. Suspects naïvely played their parts until they were arrested.

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.

This is legal, but is it legitimate? Without the F.B.I., would the culprits commit violence on their own? Is cultivating potential terrorists the best use of the manpower designed to find the real ones? Judging by their official answers, the F.B.I. and the Justice Department are sure of themselves — too sure, perhaps.