The new strategy, which has been tested in Pittsburgh, Indianapolis and Orange County, Calif., will mean that a certain number of passengers, even if they are not identified by these computerized checks, will be pulled aside and subject to an added search lasting about two minutes. Officials said passengers would be selected randomly, without regard to ethnicity or nationality.

What happens next will vary. One day at a certain airport, carry-on bags might be physically searched. On the same day at a different airport, those subject to the random search might have their shoes screened for explosives or be checked with a hand-held metal detector. "By design, a traveler will not experience the same search every time he or she flies," the summary said. "The searches will add an element of unpredictability to the screening process that will be easy for passengers to navigate but difficult for terrorists to manipulate."