When U.S. District Judge Richard Leon issued his preliminary injunction against the NSA's phone record database yesterday, part of his analysis (which I will discuss in my column tomorrow) concerned whether the collection of telephone metadata counts as a "search" under the Fourth Amendment. But Leon also considered whether such a search might be "reasonable," even without an individualized warrant, because of its usefulness in preventing terrorist attacks. That part of the analysis was pretty straightforward, since the government had presented no evidence that the database has been useful in preventing terrorist attacks:

The Government does not cite a single instance in which analysis of the NSA's bulk metadata collection actually stopped an imminent attack, or otherwise aided the Government in achieving any objective that was time-sensitive in nature. In fact, none of the three "recent episodes" cited by the Government that supposedly "illustrate the role that telephony metadata analysis can play in preventing and protecting against terrorist attack" involved any apparent urgency…. Given the limited record before me at this point in the litigation—most notably, the utter lack of evidence that a terrorist attack has ever been prevented because searching the NSA database was faster than other investigative tactics—I have serious doubts about the efficacy of the metadata collection program as a means of conducting time-sensitive investigations in cases involving imminent threats of terrorism.

Leon's conclusion on this question is striking, since you'd think the Obama administration would be highly motivated to show that the database has been crucial in saving lives. If the government cannot muster a single plausible example, how can such a massive invasion of privacy possibly be justified?

The administration has been struggling with this problem since news reports based on leaks from former NSA contractor Edward Snowden first revealed the existence of the phone record database last June. At first intelligence officials and their allies in Congress suggested that the program had been instrumental in foiling more than 50 terrorist plots. But those claims—which were immediately questioned by Sens. Ron Wyden (D-Ore.) and Mark Udall (D-Colo.), who as members of the Senate Intelligence Committee ought to know—dissolved upon close examination. Last October a ProPublica analysis concluded "there's no evidence that the oft-cited figure is accurate."

The crucial question, usually dodged by the NSA and its defenders, is whether routinely collecting everyone's phone records, as opposed to seeking specific, evidence-based court orders aimed at particular targets, has been decisive in stopping terrorist attacks. If the government has been unable to offer any examples in the last six months, it seems unlikely it ever will.