TWELVE PASSENGERS ON Northwest Airlines Flight 42, which departed Amsterdam for Mumbai on August 23, quickly aroused the crew's suspicions. Eyewitnesses reported that the 12 passengers, who were of South Asian descent, attempted to use mobile phones and pass them back and forth as the flight took off. Compounding that suspicious behavior, some of the men began walking in the aisles before the plane's seatbelt signs were off. The flight was escorted back to Dutch airspace by F-16 fighters and the passengers were arrested, but Dutch prosecutors announced the next day that "they found no evidence of a terrorist threat."

This dramatic incident comes amid what has generally been described as a rash of false alarms following the August 10 revelation of a foiled transatlantic air terror plot. Since then, at least 20 public incidents involving airline security have been reported in the United States and Europe. Recent events include a September 1 AirTran flight to San Francisco being diverted after a passenger was seen sniffing a substance in a bag, an August 29 US Airways flight from Philadelphia to Houston being diverted after a "threatening note" prompted a bomb scare, and an August 25 US Airways jet being diverted after a disruptive passenger pushed a flight attendant. The commonly accepted explanation for this spike in incidents is that airline crews and passengers are on a hair trigger. But there may also be casings and dry runs occurring, and it's difficult for an open society to guard against these exercises.

THE TRANSATLANTIC AIR PLOT that was disrupted in early August provides the latest evidence of how terrorists probe airline defenses. Intelligence sources report that at least one of the plotters took several flights between Britain and the United States with only one plausible purpose: probing weaknesses in airline security.

But even before that plot became public knowledge, there was good reason to suspect that terrorists trying to probe airline security were among the millions of people who board planes each day. Annie Jacobsen has tirelessly reported about the distressing state of airline security in both a series of articles for WomensWallStreet.com and the book Terror in the Skies: Why 9/11 Could Happen Again. Although she is not without her critics, Jacobsen impressed me as a careful and thorough journalist with an array of knowledgeable sources.

In the course of Jacobsen's investigations, a large number of airline industry personnel and Federal Air Marshals approached her with concerns about suspicious in-flight behavior by Middle Eastern men. She has catalogued many such incidents, but is often unable to report on them because of her sources' concern that they will get in trouble for speaking to the press. (Both airline personnel and Federal Air Marshals are required to enter agreements prohibiting them from doing so.) But the incidents her sources have allowed her to report on provide reason for concern.

One such incident occurred on United Airlines Flight 925, which left London for Washington, D.C. on June 13, 2004. Jacobsen recounts that nine Middle Eastern men arrived late, just minutes before the aircraft doors were set to close. Although the nine men had "arrived independently on separate itineraries from various Middle Eastern countries," after the plane took off it became obvious that at least some of them knew each other. And their behavior was unusual enough to make the crew and captain suspicious: