Scores of undercover nuclear scientists screened America's largest cities for radiation amid fears that al-Qa'ida might release a dirty bomb at an open-air event such as the New Year celebrations in Times Square.
The operation, detailed by The Washington Post , was performed in secrecy, in contrast to public efforts to thwart any attack on a passenger or cargo aircraft.
Other thana false alarm in Las Vegas, involving a homeless man who had a radium pellet, originally manufactured to treat cancer, in a duffel bag, no such bomb was detected.
Dozens of scientists searched Times Square before the New Year's Eve gathering. Others searched for radiation spikes near the Las Vegas Strip as well as at the annual Rose Bowl American football parade in Los Angeles.
The revelations cast new light on the degree of concern expressed when officials put America on an elevated code orange alert on 22 December. It is still in place.