> AFAIK these people don't commit the crimes (the FBI doesn't want the violent acts to actually occur), they just are involved in planning them.

In some cases they attempt to explode what they think are real bombs.

I've read that conspiracy charges do not always require an overt act in furtherance of the conspiracy, unless it's a government agent who suggests it. In those cases an overt act in furtherance is always required.

The cases I recall involved fake-bombs that the suspect tried to detonate. [1] Some other responses down thread include much more dubious actions by the co-conspirators which shines a much more concerning light on this FBI practice.

[1] - https://news.vice.com/article/isis-fake-bomb-terrorist-fbi-s...

Also,

http://mobile.nytimes.com/2012/04/29/opinion/sunday/terroris...