If the FBI did that, that's entrapment and might be an eligible defense. However, if the FBI assisted or even encouraged an existing plot, then I don't think it would qualify.

> Entrapment is a practice whereby a law enforcement agent or agent of the state induces a person to commit a "crime" that the person would have otherwise been unlikely or unwilling to commit

> Depending on the law in the jurisdiction, the prosecution may be required to prove beyond a reasonable doubt that the defendant was not entrapped or the defendant may be required to prove that they were entrapped as an affirmative defense.

https://en.wikipedia.org/wiki/Entrapment