Another increasingly familiar trend it seems.
The FBI had the Boston Marathon bombers practically handed to them but did nothing. They investigated the Orlando terrorist but had to drop it because of a foolish policy that they can only stay on a potential terrorist for one year. And of course, we now know they had the 9/11 terrorists under surveillance but let them go too in an epic example of FBI incompetency.
As in Europe, the authorities' hands are somewhat tied as to what they can do with these terrorists in waiting. If they are citizens, expelling them is impossible. If they are legal residents, it takes a long court fight to get rid of them, and that will result in revealing intel sources. Plus of course, this administration treats all muslims, no matter how extreme, with kid gloves and is quick to attack people who take a different view, so people are reluctant to come forward.
The inescapable conclusion is the only sure way to mitigate the risk is to severely limit immigration by muslims, particularly those from areas where proper vetting is impossible.