All their wire taps, eves dropping, billions in budgets for personnel and material, sophisticated computer systems... and yet out vaunted FBI/NSA/CIA can't find a giant red flag in writing. In this case, not only was it in writing, but another person noticed it and alerted the FBI to the message and still nothing. In fact, it appears all the FBI did was keep going back to Ben Bennight and asking if he knew Cruz.
yes, basically harassing and threatening the person bringing this threat to their attention.