A new report from the Government Accountability Office finds that the number of people on the government's Terrorist Watch List has swelled to more than 750,000. That number has been growing by 200,000 per year since the list's inception in 2003. Online discussions of the report have reflected a common misconception: the terrorist watch list is not the same as the no-fly list, which gets you banned from airplanes altogether, or the selectee list, which the TSA uses to decide who gets extra screening in the security line. According to the GAO, the standards for adding someone to the no-fly and selectee lists are more stringent than the standards for adding individuals to the broader watch list, and as a result, those lists are much shorter than the full watch list. The report does not give details on the size of these lists, but reports have pegged the lists at 44,000 and 75,000 people, respectively.

The report also found ongoing problems with people on the no-fly list being permitted to fly. On international flights, passenger lists are reviewed twice, once before a plane takes off and a second time afterwards. The GAO found that a number of individuals on the no-fly lists had been allowed to board airplanes, and their identities had only been caught during the second round of reviews. Domestic flights, on the other hand, are not subject to a second review, making it impossible to know how many members of the no-fly list have managed to fly domestically.

An international watch list seems like a sensible precaution for reviewing visa and immigration requests. Keeping terrorists out of the country is a worthwhile goal, and it makes sense for the government to keep a master list of individuals with documented links to terrorist activities. However, the list would be more useful if it were a lot shorter. It seems unlikely there are anywhere close to 750,000 people plotting terrorist acts around the world. A list that large ensures a high false-positive rate, wastes law-enforcement resources, and ensures that border control officials will not take the watch list as seriously as they should when an actual terrorist shows up.

On the other hand, it's difficult to see what purpose is served by a domestic no-fly list. If government officials have concrete evidence that an American person is engaged in terrorist-related activities, then the government should be doing a lot more than putting that individual on a no-fly list. They should be actively investigating the individual, tapping his phone, reading his email, monitoring his financial transactions, and generally gathering the evidence required to either clear his name, deport him, or arrest him.

If, on the other hand, the government doesn't have enough evidence of terrorist ties to justify starting an investigation against an individual, then it's unreasonable, not to mention a waste of law enforcement resources, to ban him from flying on airplanes or subject him to heightened scrutiny every time he goes to an airport. The sheer number of people on the selectee list and the high rate of false positives may be one reason that screeners do a legendarily bad job finding simulated weapons in security tests. The resources now spent on screening tens of thousands of selectees—most of whom turn out to be false positives—would be far better spent on additional FBI agents to do in-depth investigations of people with actual terrorist ties.

The GAO report makes a variety of recommendations for making the watch list process more effective, but they do not address the serious constitutional issues raised by the domestic use of such a list. As we've noted before, the legal protections normally available to individuals stopped by government officials for search and questioning are largely absent when dealing with the TSA and other parts of the anti-terrorism bureaucracy. If you're mistakenly placed on the terrorist watch list, there's no easy way for you to even learn that you've been placed on the list, to say nothing of getting yourself removed. Instead, the GAO suggests that "additional screening opportunities remain untapped," suggesting that the constitutional concerns will only get worse in the coming years.

Unfortunately, the TSA and other homeland security agencies spend a lot of time engaging in what Bruce Schneier calls security theater. There's little reason to think the ever-expanding no-fly list actually makes anyone safer, but its existence makes some travelers feel safer. And that perception is all the TSA needs to justify continuing to harass the traveling public at taxpayer expense.