We are confident that, in light of the experience of the American colonists with the abuses of the British Crown, the Framers of our Constitution would have disapproved of the arrest, detention, and harsh confinement of a United States citizen as a "material witness" under the circumstances, and for the immediate purpose alleged, in al-Kidd’s complaint. Sadly, however, even now, more than 217 years after the ratification of the Fourth Amendment to the Constitution, some confidently assert that the government has the power to arrest and detain or restrict American citizens for months on end, in sometimes primitive conditions, not because there is evidence that they have committed a crime, but merely because the government wishes to investigate them for possible wrongdoing, or to prevent them from having contact with others in the outside world. We find this to be repugnant to the Constitution, and a painful reminder of some of the most ignominious chapters of our national history.

The whole opinion is here.