I don't think that it can conceivably be possible to imprison anybody before a crime is committed. Such a thing seems to violate the Universal Declaration of Human Rights on several counts and would therefore be opposed by the United Nations. Article 9 of the UDHR states, "No one shall be subjected to arbitrary arrest, detention or exile." "Precrime" law enforcement would be doing just that. Articles 10 and 11 states that everyone has a right to an "impartial tribunal" and that "Everyone charged with a penal offence has the right to be presumed innocent until proved guilty...". Imprisoning someone before a crime is committed demonstrates neither impartiality or presumption of innocence.