The CIA, working with counterparts in the Middle East, earlier this week halted the latest al Qaeda terrorist plot to bomb aircraft bound for the U.S. The planned attack, which would have come from explosives worn under a passenger's clothing, is reminiscent of the so-called underwear bomb worn by an al Qaeda operative in the failed attempt by Umar Farouk Abdulmutallab to bring down a Detroit-bound passenger airliner on Christmas Day 2009. The latest underwear bomb found through the covert CIA operation is thought to be the work of Ibrahim Hassan al Asiri, who designed the original device.

Although the plot was disrupted before a particular airline was targeted and tickets were purchased, al Qaeda's continued attempts to attack the U.S. speak to the organization's persistence and willingness to refine specific approaches to killing. Unlike Abdulmutallab's bomb, the new device contained lead azide, an explosive often used as a detonator. If the new underwear bomb had been used, the bomber would have ignited the lead azide, which would have triggered a more powerful explosive, possibly military-grade explosive pentaerythritol tetranitrate (PETN).

Lead azide and PETN were key components in a 2010 plan to detonate two bombs sent from Yemen and bound for Chicago—one in a cargo aircraft and the other in the cargo hold of a passenger aircraft. In that plot, al-Qaeda hid bombs in printer cartridges, allowing them to slip past cargo handlers and airport screeners. Both bombs contained far more explosive material than the 80 grams of PETN that Abdulmutallab smuggled onto his Northwest Airlines flight.

With the latest device, al Asiri appears to have been able to improve on the underwear bomb supplied to Abdulmutallab, says Joan Neuhaus Schaan, a fellow in homeland security and terrorism for Rice University's James A. Baker III Institute for Public Policy. This is just the latest in the "very serious cat-and-mouse game" that terrorists play with those trying to stop them.

"In this particular case it's interesting to see the way the terrorists were trying to use resistance to [Transportation Security Administration] procedures as part of an attack," Schaan says. After Abdulmutallab's attempt a few years ago, the TSA put in place new procedures and technologies to prevent someone else from smuggling explosives on board an aircraft in their clothing. Shortly thereafter the general public took offense to these new security methods, and the TSA was required to rethink it policy, she adds.

The joint CIA–Saudi intelligence operation to stop this latest attack, orchestrated by Yemen-based Sunni terrorist group al Qaeda in the Arabian Peninsula (AQAP), coincides with several other significant terrorism-related developments of the past week. In addition to the recent one-year anniversary of former al Qaeda leader Osama bin Laden's assassination by U.S. military forces in Pakistan, a CIA drone strike earlier this week in Yemen killed AQAP head of operations, Fahd Mohammad Ahmed al Quso, an alleged planner of the 2000 terrorist attack on the USS Cole.

On Monday al Qaeda released a hostage tape featuring former American Peace Corps and U.S. Agency for International Development (USAID) official Warren Weinstein, who was kidnapped last August in Pakistan. Meanwhile, the military trial of accused 9/11 planner Khalid Sheik Mohammed and four others has begun in Guantanamo Bay, Cuba.

Scientific American spoke with Schaan about al Qaeda's continued attempts to take down airliners traveling to the U.S., the terrorist organization's focus on exploiting cultural norms to reach their targets and the most successful approaches to stopping terrorist plots.

[An edited transcript of the interview follows.]

How significant was the discovery of this plot to blow up a U.S.-bound airliner with an improved version of an underwear bomb?
It further illustrates the fact that even though we've killed Osama bin Laden, Anwar al Awlaki [a key AQAP operative who died in a September 2011 drone air strike] and several other al Qaeda leaders, we have not stopped the threat they pose.

Why would al Qaeda be trying to develop a new underwear bomb, after the first attempt failed?
An underpants bomb is worn under a person's clothes, just like a diaper. The people behind these plots understand not only the TSA's security procedures but how they are tolerated—or in this case not tolerated—by travelers in the Western world, and the terrorists used this knowledge to design their attacks. The plotters might not have gone back to an underpants bomb if the TSA had continued with the more intensive screening procedures it had in place.

Although Abdulmutallab was stopped, how was he able to get as far as he did with his planned attack?
Abdulmutallab began his journey in Lagos, Nigeria, on December 24, and the initial security screening would have occurred there. The Lagos airport has had a well-known reputation for lax security. PETN, [the explosive] which Abdulmutallab tried to use, is widely available. It can easily be detected if checked by dog, swab or "puffer" machine, but it's hard to detect in a sealed container. In addition, passengers are most often checked only by magnetometers. In the case of Abdulmutallab, he attempted to detonate the device by injecting a chemical into it after he had gotten onto the airplane, but the attempt was unsuccessful.

The new bomb supposedly contained no metal. Does this mean it would have passed through airport detectors unnoticed?
Yes, that would be the case with a magnetometer, if the bomb did not have any metal wiring, which tends to be picked up. Airport security screeners also swab people, luggage and handbags, looking for a variety of substances, including drugs and explosive chemicals. Although they likely wouldn't swab someone's underwear, the chemical might have gotten on that person's luggage and clothes without them realizing it. Chemical analysis may have been a way to detect this type of explosive but airport screeners don't swab every bag.

Would backscatter x-ray machines or millimeter wave scanners deployed at many airports have been effective in spotting the new underwear bomb?
Those scanners are looking for anomalies on a person's body, in particular something that is hard or dense. It depends on the density of the bomb materials, what the ingredients are and the form they took.

[Backscatter picks up the radiation bounced back from the passenger's body and any objects the person may be concealing. Elements such as carbon, oxygen, hydrogen and nitrogen—common ingredients of explosives—create a strong scattering effect visible in images that operators monitor on a screen yet are discernible from the organic molecules in the human body. Millimeter waves, which are shorter than microwaves but longer than infrared, create a revealing picture that can highlight items and determine the precise chemical makeup of an object. Although backscatter and millimeter wave scanners might have given screeners cause to be suspicious of an underwear bomb, neither are used in all airports, and passengers concerned about exposure to radiation can often opt to instead be frisked by security personnel.]

In addition to the original underwear bomb and the 2010 attempts to pack explosives as cargo on aircraft, what other tactics has al Qaeda tried in their search for effective methods of attack?
[Ibrahim Hassan] al Asiri had his own brother place PETN inside his colon and then told him to surrender to the counter-terrorism chief in person and request entry into Saudi Arabia's terrorist rehabilitation program. In August 2009 Abdullah Hassan al Asiri tried to assassinate Saudi Arabia Interior Minister Prince Muhammed Nayef bin Abdul Aziz al Saud, although the PETN explosion ended up killing him and only slightly injuring al Saud. Ironically, al Saud was protected from the full brunt of the blast by al Asiri's body. Less than half a year later, Abdulmutallab tried the first underpants bomb.

In each case, the bomb-makers take into account cultural norms. For example, the Taliban developed a turban bomb, which was employed to assassinate Burhanuddin Rabbani, a former Afghan president and head of the High Peace Council. The tactic was used because Afghans are reluctant to request the removal of a turban.

What has been the most effective means of disrupting terrorism attacks?
As with bombs that were being sent from Yemen to Chicago as cargo, this latest plot was discovered using human intelligence rather than screening procedures and technologies. These plans were disrupted because of proactive mechanisms put in place to stop terrorism rather than defensive approaches such as screening.