Disclaimer: I am not a lawyer.
My wife and I are planning to travel Thanksgiving Day, and we've both been keeping track of the TSA Rapiscan story. This morning I asked myself, "What if a traveler pleads the Fourth Amendment?" I've been doing a little digging since then, and I thought it would be valuable to share my findings here.
According to the TSA's website, the legal basis for the Administration's search authority is United States v. Davis, 482 F.2d 893, 908 (9th Cir. 1973):
Do contracted screeners draw their authority from the Aviation Transportation and Security Act PL107-71 (ATSA), or do state and local governments have to codify ATSA to establish their authority to perform Administrative Searches? If so, what if there is a conflict with the State Constitution (e.g., random vehicle searches during heightened alert conditions)? Will the Screening Standard Operating Procedures be modified to accommodate these conflicts?
Even prior to the passage of ATSA and the Federalization of the screening work force, Federal courts upheld warrantless searches of carry-on luggage at airports. Courts characterize the routine administrative search conducted at a security checkpoint as a warrantless search, subject to the reasonableness requirements of the Fourth Amendment. Such a warrantless search, also known as an administrative search, is valid under the Fourth Amendment if it is "no more intrusive or intensive than necessary, in light of current technology, to detect weapons or explosives, " confined in good faith to that purpose," and passengers may avoid the search by electing not to fly. [See United States v. Davis, 482 F.2d 893, 908 (9th Cir. 1973)].
So here's United States v. Davis. Basically, Davis attempted to board a plane while hiding a gun in his briefcase. He was confronted by a law enforcement officer who searched his briefcase and discovered the gun. Davis was charged with attempting to board an aircraft while carrying a concealed weapon. His unsuccessful defense was on Fourth Amendment grounds. The 9th Circuit basically found that "administrative" searches, which have a goal other than criminal investigation, are acceptable under the Fourth Amendment with caveats.
I'd suggest you read the whole thing if you're so inclined, but the crux of my post is TSA's claim of performing an "administrative" search. Here are the relevant limitations the decision provides (footnotes removed):
54 One important caveat should be stressed, however. To meet the test of reasonableness, an administrative screening search must be as limited in its intrusiveness as is consistent with satisfaction of the administrative need that justifies it. It follows that airport screening searches are valid only if they recognize the right of a person to avoid search by electing not to board the aircraft.55 It is difficult to see how the need to prevent weapons and explosives from being carried aboard the plane could justify the search of a person who had elected not to board. Perhaps it could be argued that a compelled search might lead to the apprehension of a potential hijacker, eliminating or at least reducing the chance that he would try again. Compared to the degree of additional intrusiveness that compulsory searches involve, however, this possibility seems so slight as to be inconsequential. The risk of successful hijacking is not enhanced by allowing a potential passenger to avoid a search on a particular occasion by electing not to fly. Airport screening searches, as carried out in this case and as currently employed nationwide, are not selective. A prospective passenger who elects not to fly on an earlier flight is, like all other passengers, certain to be subjected to a search before he can board a later flight.56 It is significant that the regulations establishing the airport search program do not authorize or require compelled searches.57 Since a compelled search of persons who elect not to board would not contribute to barring weapons and explosives from the plane, it could serve only the purpose of apprehending violators of either the criminal prohibition against attempting to board an aircraft while carrying a concealed weapon, 49 U.S.C. Sec. 1472(l), or some other criminal statute. Such searches would be criminal investigations subject to the warrant and probable cause requirements of the Fourth Amendment.58 In sum, airport screening searches of the persons and immediate possessions of potential passengers for weapons and explosives are reasonable under the Fourth Amendment provided each prospective boarder retains the right to leave rather than submit to the search.