What is a preventative jurisprudence?

Our criminal law is founded on punitive measures: only after a crime is committed (or attempted) do we punish the offender. Now, terrorism, with its threat of mass casualties -- be it from a suicide bombing or a nuclear attack -- poses the question: can we afford to wait? And, if we decide that we can't wait for the bomb to go off to take the criminal to court, how do we deal with a threat responsibly? What kind of legal structure do we need to adequately and appropriately address big threats?