John Mueller suggests an alternative to the FBI's practice of encouraging terrorists and then arresting them for something they would have never have planned on their own:
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...