When it was determined she knew of his plans and “tried” to stop it, but told no one she should have been detained immediately for further investigation and possible prosecution.
18 U.S. Code § 4 - Misprision of felony
Whoever, having knowledge of the actual commission of a felony cognizable by a court of the United States, conceals and does not as soon as possible make known the same to some judge or other person in civil or military authority under the United States, shall be fined under this title or imprisoned not more than three years, or both.She confessed to the FBI that she had knowledge of the attack. The FBI had solid grounds to take her in.
That they didn't tells you all that you need to know.