(CNN) -- Three months before he was killed by a U.S. drone strike, Fahd al Quso, one of al Qaeda's top operatives in Yemen, spoke at length to a local journalist. He was asked why al Qaeda in the Arabian Peninsula had stopped plotting against the United States. Was it because all efforts were devoted to an internal project?
"The war didn't end between us and our enemies. Wait for what is coming," al Quso replied.
It seems al Quso, the head of the group's external operations, wasn't bluffing, after the recent discovery of a device designed to be carried aboard an airliner by a suicide bomber without detection.
U.S. officials describe the device as an evolution of the bomb smuggled aboard a U.S.-bound plane on Christmas Day 2009 by a young Nigerian, Umar Farouk AbdulMutallab.
On that occasion, according to bomb disposal experts, the passengers on board Northwest Airlines Flight 253 from Amsterdam to Detroit were very lucky. But al Qaeda's master bomb-maker in Yemen, a young Saudi named Ibrahim al-Asiri, was not deterred. Within months, he had designed a device to be hidden inside a printer and sent as air freight to the United States.
Only a last-minute intelligence tip from Saudi Arabia led to the discovery of two bombs hidden inside laser printers. Dispatched from Yemen's capital, Sanaa, one had reached an airport in England and was so well-disguised that the first examination of the printer by British police failed to detect the bomb.
An evolving capability
It's not clear whether al-Asiri built the most recent device, nor have U.S. officials described how it differed from the Christmas Day bomb. One official said that like the earlier device, it was "non-metallic" and therefore much harder for airport security scanners to detect.
"It is clear that AQAP is revamping its bomb techniques to try to avoid the causes of the failure of the 2009 device," the official said.
This time, it seems that good intelligence rather than luck foiled the plot. According to White House Chief Counterterrorism Adviser John Brennan, the device never got near an airport. That implies at least that if it left Yemen, it did so overland. But as another official acknowledged, there is no knowing whether other similar bombs may be out there.
And as Brennan appeared to acknowledge, intelligence is a key asset when airport security systems are not foolproof in the face of an adaptive adversary.
"Being able to take action before any of these IEDs can make their way to an airplane or airport is instrumental in terms of being able to disrupt these types of attacks before they get under way," he told NBC on Tuesday.
Al Qaeda's zone of immunity
Over the past year, al Qaeda in the Arabian Peninsula has carved out what might be termed a zone of immunity in southern Yemen, far beyond the reach of an ever-weakening state. According to one U.S. counterterrorism official, "Those territorial gains have allowed the group to establish additional training camps."
Saudi counterterrorism officials believe that the group has taken a back seat in the fighting in Yemen and has instead taken advantage of the breathing space opened by jihadist advances to build up its cell structure and a network of safe houses, according to Mustafa Alani, director of terrorism and security studies at the Gulf Research Center.
The most recent edition of the group's online magazine, Inspire, published last week, boasted that a modest laboratory in a rural area of Yemen had produced both the Christmas Day and printer bombs.
Now, the article continued, "AQAP has obtained a large deal of chemicals from military laboratories after they conquered Zinjibar (a town on the Arabian Sea) and other towns and cities." The modest lab had been transformed into a modern one, with no "wearisome measures" needed to obtain large amounts of chemicals for explosives.
While there is probably a good deal of bluster in the claims made in Inspire, intelligence officials say al Qaeda's bomb-makers should not be underestimated.
The design of the Christmas Day bomb was ingenious, according to counterterrorism officials. A specially sewn pouch in AbdulMutallab's underwear contained the main PETN explosive charge, which was connected to a detonator. The initiation for the device was a syringe in his underwear filled with two easily obtainable chemicals: potassium permanganate and ethylene glycol.
PETN is a white, odorless powder than cannot be detected by most X-ray machines. AbdulMutallab revealed to the FBI in his initial interview that he wore the underwear device on several flights during an almost three-week journey through Africa before traveling from Lagos to Amsterdam.
As Northwest 253 made its final approach to Detroit, he plunged the syringe, mixing the two chemicals and setting them afire. According to the prosecution, this flame set off the detonator, but the PETN main charge was not detonated. Instead, some of it started burning, creating a fireball on AbdulMutallab's lap.
An explosives expert says that a likely explanation for the failure of the underwear device to fully detonate was wear and tear during AbdulMutallab's lengthy transit through Africa. When the device was later examined, al-Asiri's fingerprints were found on it.
And it wasn't just innovative bomb-making that gave Western intelligence pause in the aftermath of the Christmas Day plot. It emerged at AbdulMutallab's trial that the FBI found a slip of paper in his shoes on which was written an encryption code. According to the prosecution, this was a password for encryption software used by al Qaeda given to him so he could communicate online with his handlers before boarding Flight 253.
Troublingly for Western counterterrorism officials, the documents were encrypted using software easily downloaded from the Internet. The messages do not appear to have been intercepted by Western intelligence agencies.
According to security sources, the printer bombs delivered to UPS and FedEx offices in Sanaa in October 2010 were a further advance. Even when one of the suspect printers was isolated in the UK (after two flights), the bomb was not detected by specially trained dogs or an X-ray scanner. There were 400 grams of the high explosive PETN inside the ink cartridge.
"The toner cartridge contains the toner which is carbon-based and that is an organic material. The carbon's molecular structure is close to that of PETN," al Qaeda in the Arabian Peninsula boasted later.
The bomb had been timed to explode when the plane scheduled to carry it would likely have been over the eastern seaboard of the United States, according to UK authorities. But the bomb squad had inadvertently defused the device when they had lifted the printer cartridge out.
A few weeks after the incident, U.S. Sen. Susan Collins asked Transportation Security Administration chief John Pistole whether the bombs would have been detected by the country's current security system.
"In my professional opinion, no," Pistole replied.
It was the most sophisticated al Qaeda device that Western counterterrorism officials had ever seen, and they said it had the potential to bring down a plane.
One of al Qaeda's aims is -- and long has been -- to force the West to spend ever larger sums on aviation security. "(Our goal was to) force upon the West two choices: You either spend billions of dollars to inspect each and every package in the world or you do nothing and we keep trying again," al Qaeda in the Arabian Peninsula said after the package bomb plot.
Small amounts of PETN in check-in luggage, checked bags, and small cargo packages can be detected by the latest generation of trace detection equipment and advanced X-ray systems.
But Kevin Riordan, technical director at Smiths Detection, a leading producer of explosive detection systems, said last year that al Qaeda can take steps to hide devices from even the latest equipment.
"We'd have to say there is always a way through," he said. "The risk is never removed totally."
The TSA responded to the the underwear bombing attempt by introducing a new generation of body scanners at the most busy U.S. airports, in an iniative that has proven controversial with the U.S. public. More than 570 are now in operation at 130 U.S. airports.
Not all passengers at these airports pass through the detectors. These scanners use advanced imaging technology (known as AIT) to detect anomalies in mass. According to a former senior U.S. security official, the machines have boosted security, but are no panacea. The former official told CNN the machines have a good chance of detecting PETN inside an underwear-type device -- perhaps in the order of 80 to 90% -- but cannot guarantee detection.
The former official said that due to their high cost, the machines had yet to be installed at many smaller feeder airports in the United States and that overseas airports were also lagging behind in introducing body scanners. While some airports in Europe and other major international hubs had introduced AIT technology, many overseas airports were still relying on a combination of physical search and walk-through metal detectors.
The former official told CNN that AIT body scanners would have little chance of detecting so called "body-bombs." In summer 2009, al Asiri fitted out his own brother with a device designed to be inserted in his rectum in a failed attack on the head of Saudi counterterrorism.
Insiders in the field of explosive detection are not confident that current detection machines can guarantee detection of all the different emerging devices being developed by bomb-makers like al Asiri, the former official said.
The former official expected the TSA to react to the recent threat stream by introducing more physical searches. The former official said they believed the TSA would also need to rely more heavily on behavioral profiling in the future. But given the lack of guarantees when it comes to detection, the former official stressed the hope of U.S. counterterrorism services was always to thwart plots through intelligence gathering before an operative had an opportunity to try to board a plane.
The Pakistan connection?
Just how al-Asiri became an expert bomb-maker is not clear, though he did study chemistry at a Saudi university. But two years ago, the governor of Marib province in Yemen, Ahmad al Maseeri, told an interesting story. He claimed that a Pakistani expert had trained members of al Qaeda in the Arabian Peninsula on ways to build and detonate explosive devices. The Pakistani died at some point in 2009, al Maseeri told a Yemeni newspaper, perhaps in an explosives accident.
The account has never been confirmed and the bomb-maker never identified. By then, al-Asiri was becoming an accomplished bomb-maker
"He is, in fact, undoubtedly one of, if not the largest threats that we face right now," a senior U.S. counterterrorism official said. "He's smart, determined and quite secretive about his activities."
Saudi Arabia's counterterrorism service believes that al-Asiri has trained several apprentices in how to make sophisticated PETN-based bombs.
"They understand that Asiri is going to be killed or captured one day," Alani said. "We're talking about a new generation of very skillful bomb builders and very committed people."