We've reached the time where we now execute before crimes are committed, like in the movie Minority Report. As we can see, this isn't just applied to external enemies either. The government's chief concern is not defending the life of its citizens, but defending itself, and thus even citizens can now be labelled "terrorists" and punished without having actually broken any just laws.
So, if we have solid evidence that a crime (like a terrorist attack) is going to happen, should we wait until such crime actually happens?
The idea that we know what someone will do with their free will is crap. But if you know someone is conspiring to commit a crime, arrest them for conspiring.
In your scenario there was solid evidence, so use it.
But do you arrest someone for murder if you think they will murder, but haven't? No, of course not.
If they conspire, charge them; if they attempt, charge them; if they incite, charge them with that; and if they murder, charge them with that.