May 28, 2024

Walter Murphy Stopped at Airport: Another False Positive

Blogs are buzzing about the story of Walter Murphy, a retired Princeton professor who reported having triggered a no-fly list match on a recent trip. Prof. Murphy suspects this happened because he has given speeches criticizing the Bush Administration.

I studied the no-fly list mechanism (and the related watchlist) during my service on the TSA’s Secure Flight Working Group. Based on what I learned about the system, I am skeptical of Prof. Murphy’s claim. I think he reached, in good faith, an incorrect conclusion about why he was stopped.

Based on Prof. Murphy’s story, it appears that when his flight reservation was matched against the no-fly list, the result was a “hit”. This is why he was not allowed to check in at curbside but had to talk to an airline employee at the check-in desk. The employee eventually cleared him and gave him a boarding pass.

(Some reports say Prof. Murphy might have matched the watchlist, a list of supposedly less dangerous people, but I think this is unlikely. A watchlist hit would have caused him to be searched at the security checkpoint but would not have led to the extended conversation he had. Other reports say he was chosen at random, which also seems unlikely – I don’t think no-fly list challenges are issued randomly.)

There are two aspects to the no-fly list, one that puts names on the list and another that checks airline reservations against the list. The two parts are almost entirely separate.

Names are put on the list through a secret process; about all we know is that names are added by intelligence and/or law enforcement agencies. We know the official standard for adding a name requires that the person be a sufficiently serious threat to aviation security, but we don’t know what processes, if any, are used to ensure that this standard is followed. In short, nobody outside the intelligence community knows much about how names get on the list.

The airlines check their customers’ reservations against the list, and they deal with customers who are “hits”. Most hits are false positives (innocent people who trigger mistaken hits), who are allowed to fly after talking to an airline customer service agent. The airlines aren’t told why any particular name is on the list, nor do they have special knowledge about how names are added. An airline employee, such as the one who told Prof. Murphy that he might be on the list for political reasons, would have no special knowledge about how names get on the list. In short, the employee must have been speculating about why Prof. Murphy’s name triggered a hit.

It’s well known by now that the no-fly list has many false positives. Senator Ted Kennedy and Congressman John Lewis, among others, seem to trigger false positives. I know a man living in Princeton who triggers false positives every time he flies. Having many false positives is inevitable given that (1) the list is large, and (2) the matching algorithm requires only an approximate match (because flight reservations often have misspelled names). An ordinary false positive is by far the most likely explanation for Prof. Murphy’s experience.

Note, too, that Walter Murphy is a relatively common name, making it more likely that Prof. Murphy was being confused with somebody else. Lycos PeopleSearch finds 181 matches for Walter Murphy and 307 matches for W. Murphy in the U.S. And of course the name on the list could be somebody’s alias. Many false positive stories involve people with relatively common names.

Given all of this, the most likely story by far is that Prof. Murphy triggered an ordinary false positive in the no-fly system. These are very annoying to the affected person, and they happen much too often, but they aren’t targeted at particular people. We can’t entirely rule out the possibility that the name “Walter Murphy” was added to the no-fly list for political reasons, but it seems unlikely.

(The security implications of the false positive rate, and how the rate might be reduced, are interesting issues that will have to wait for another post.)


  1. “If he was stopped before the speech, then it wasn’t the speech.”

    Not necessarily. If there was a widespread expectation that his speech would be a scathing indictment of Bush or whatever, then there’s more motivation to try to prevent him reaching the venue and making the speech than there is to harass him afterward.

    On the other hand, presumably if DHS really wanted him detained instead of making the speech, he would have been. The conspiracy theory seems highly unlikely, especially in light of there being an IRA mad bomber on record as having used a very similar name.

  2. The only way to know is to look at a differential.

    If he was stopped before the speech, then it wasn’t the speech. The Walter Murphy in the IRA has been in the IRA for at least 20 years, so should hardly be a new addition, since the IRA stood down a good few years back now.

    That he wasn’t stopped on the way back might simply be because the return journey was booked at the same time as the outward journey, and hence the system is “smart” enough to realise that if they got one way, they can come back without risk. Of course, that’s a wide open exploit right there, if true.

    I’m sure there are millions of Americans who don’t fly any more. I hate flying now (at least with public transport type) as you are treated like cattle, and the new insane rules to bilk passengers out of more money by stopping them taking a bottle of water with them, or any other liquids, past security, for an imaginary bomb threat someone invented, pisses me off.

    If you believe the liquid bomb syth. idea, you know nothing about chemistry. The easy way to stop dodgy chemicals going through as hand luggage? Simply make the carrier drink some of the water in front of the guard. Anyone dumb enough to try drinking battery acid will soon be filtered from the gene pool. Then and only then will I believe that all this isn’t to make more money in the cafes behind security check-in.

  3. There is a walter F murphy a member of the IRA and he is on list. A man with a similar name in Santa Fe is stopped each time he flies. Just a coomon error no big conspiracy on the part of bad guys…..

  4. cisco kidd says

    I guess all you crack pots think there is UFOS also that they are hiding also

  5. I’m also a false positive every time I fly. I’m unable to use electronic or curbside checkin, and must always go the the ticket counter to check in for my flight, which can add significant delays, especially since I never check luggage.

  6. There are other possible factors than dissent that may have flagged Prof. Murphy. Without knowing the criteria, it’s hard to make an assessment of cause.

    I was selected for the full security treatment last year because (I suspect) of the combination of

    1) The ticket was purchased less than 24 hours in advance
    2) My employer paid for it, not myself, so name on credit card did not match name of passenger.
    3) I was a foriegn national working in the US

    The consequence was minor – 2 more long lines, no prescreening information so curbside check wait was for naught, sent inside once I hit head of line. Also a full “empty everything out” inspection of luggage.

    I also experienced it once in Canada, where the security person had a stack of paper slips, some white, some yellow, and they handed each out to those who passed, dealt off the top – if you got a yellow one, you had a full inspection.

    Get there early and be prepared for a wait is always good advice re air travel these days.


  7. Brian,

    What math do you want me to do?

    It’s clear that the police have sometimes gone too far in monitoring, harrassing, and even arresting people who did nothing more than dissent. It’s clear, too, that rhetoric equating dissent with disloyalty to the country helped to enable those actions.

    But none of that establishes that this particular guy was singled out at the airport because of his views.

  8. Murphy reports that he wasn’t similarly detained on his return leg, which may argue against the false-match hypothesis, unless the false matches are probabilistic in some way, or there’s a method for (temporarily) causing a particular false match not to come up.

    But for purposes of chilling speech and political activism, the perception that publicly dissing the administration will get you on the list is sufficient. (And if deliberately or negligently aided by the TSA or its assigns, unlawful.)

  9. Come on, do the math!! How can people refuse to acknowledge the obvious — of course it was political revenge. You probably would have also defended the police during their year long denial of photographing or interrogating demonstraters just for demonstrating, until eventually the truth came out. Or the undercover investigators infiltrating peace groups. Promoting peace is acting suspicious? It’s traitorous? We need some truth about this crazy no-fly list — who makes it, how does one get on it, does it do any good? It’s bogus, man.

  10. Note, too, that Walter Murphy is a relatively common name, making it more likely that Prof. Murphy was being confused with somebody else.

    indeed, when i saw this post’s headline, i wondered aloud if the commission of discofication against classical works might not have been enough to land ol’ WM on a watchlist.

  11. The correct explanation is, of course, Murphy’s Law.

  12. Did Professor Murphy fly before giving his speech? If so, did his name match one on the no-fly list at that time? If not one possible explanation is that the matching name was recently added to the list. Coincidence? Possibly.

    I think it says more about the state of our government that many (most?) people have no problem believing that a name would be added to the list to harass those who criticize the Bush Administration.

    Bruce Schneier on the no-fly list:
    It’s a list of people so dangerous that they cannot be allowed to fly
    under any circumstance, yet so innocent that they cannot be arrested…
    even under the provisions of the PATRIOT ACT.

  13. Well,

    As the list and process is shrouded in secrecy one could easily conclude this was not a mistake. Even if it was a false positive, the system has been designed to produce many false positives while not providing security, thus it can be argued there are no a mistakes made in good faith by the system.

    My conspiracy theorist belief is the entire TSA setup, no fly lists included, is meant to condition the American people into accepting progressively more intrusive infringements on their liberties by the federal government.