The experience with another case can be taken to suggest that there could be an alternative, and far less costly, approach to dealing with would-be terrorists, one that might generally (but not always) be effective at stopping them without actually having to jail them.
It involves a hothead in Virginia who ranted about jihad on Facebook, bragging about how “we dropped the twin towers.” He then told a correspondent in New Orleans that he was going to bomb the Washington, D.C. Metro the next day. Not wanting to take any chances and not having the time to insinuate an informant, the FBI arrested him. Not surprisingly, they found no bomb materials in his possession. Since irresponsible bloviating is not illegal (if it were, Washington would quickly become severely underpopulated), the police could only charge him with a minor crime — making an interstate threat. He received only a good scare, a penalty of time served and two years of supervised release.
That approach seems to have worked: the guy seems never to have been heard from again. It resembles the Secret Service’s response when they get a tip that someone has ranted about killing the president. They do not insinuate an encouraging informant into the ranter’s company to eventually offer crucial, if bogus, facilitating assistance to the assassination plot. Instead, they pay the person a Meaningful Visit and find that this works rather well as a dissuasion device. Also, in the event of a presidential trip to the ranter’s vicinity, the ranter is visited again. It seems entirely possible that this approach could productively be applied more widely in terrorism cases. Ranting about killing the president may be about as predictive of violent action as ranting about the virtues of terrorism to deal with a political grievance. The terrorism cases are populated by many such ranters — indeed, tips about their railing have frequently led to FBI involvement. It seems likely, as apparently happened in the Metro case, that the ranter could often be productively deflected by an open visit from the police indicating that they are on to him. By contrast, sending in a paid operative to worm his way into the ranter’s confidence may have the opposite result, encouraging, even gulling, him toward violence.