Maybe the FBI is radicalizing these people.
Remember the alerts that were issued about some of the 911 hijackers being inside the USA that went nowhere ?
If I'm a radical Muslim, and I think "My best bet to get to paradise is jihad, and I'll do it one of these days (really!) but I just don't have the courage right now ..."
And then the FBI picks me up and talks with me? I start thinking my time is short. I start thinking I will be deported or put in prison. I start thinking that my best chance at paradise might be snatched away from me.
I start making plans to drive a truck where I'm not supposed to drive a truck.
My takeaway: If the FBI talks to someone, one goal of the FBI should be to get them out of the country. The suspect's main goal should be to justify their presence here and to convince the FBI that they would absolutely never do anything bad. That should be a very tough sell to the FBI. These people need to go if there is any doubt. Now, if they are US citizens, it's different. If not a citizen? Bye-bye.
Question the people you want to question and then deport the people you wanted to question. Just because.