How far is too far?

This here is a bit old, but I've been meaning to mention it.

In April, a militia-group was rounded up and placed under arrest for planning to shoot up a bunch of police officers. There was a debate over whether to release them or not, since their case was mostly based on speech alone, with no definitive plan of action to show intent.

On the other hand, if you arrest a group of radical militants that meet together to talk about how the government holds tyranny over their lives, and that the only way to stop it is to become martyrs in a grand gesture of multiple murders, then you let them go free briefly before their trial... does this not give them incentive to believe their views and furthermore give them no reason to not act on them?

I believe in the protections of free speech, and I don't believe in locking people up for no good reason... but I also recognize a need for public safety.

I just don't know how to think on this one. What about you?