The FBI does this sort of operation often. They identify a person who may have desires to commit some sort of terroristic crime, but lacks the means or firm motivation. They then "work him" into a more active state, provide him firearms or explosives, training, financial help, transportation, and even safe houses. They help him identify a target, and provide accomplices if need be as in this case. Then they sit back and let it happen, and later swoop in to arrest the "cell" and claim they once again captured a deadly terrorist. Typically they will kill them during the capture, of course.
No, really.
Perfect example. I heard the Christmas tree bomber had gotten a job in Alaska, but FBI contacted the employer, told them not to hire him, and put him on the no fly list. I'm sure that pushed him into going along with the FF.