Late last October, a pair of innocuous packages were dropped off at a courier’s office in Sanaa, Yemen, for shipping to an address in Chicago. Hours later, the two brown boxes - stuffed with books, clothing, and brand new laser printers - were loaded into the cargo hold of passenger planes bound for Dubai and Doha on the first leg of their journey to the United States.

What the hundreds of passengers on those flights did not know was that ingeniously concealed in the printer cartridges inside those printers were explosive devices containing a white powdery chemical known as PETN.

Al Qaeda had found a potentially lethal flaw in aviation security. Conventional single beam X-ray machines rarely detect PETN. It was the same substance that had been smuggled aboard a U.S.-bound airliner by the alleged "underpants bomber," Umar Farouk Abdulmutallab, the previous Christmas. Western counter-terrorism sources say that chemically, the two batches were virtually identical, suggesting they’d been made by the same bomb-maker.

The printer bombs were timed to explode on the last leg of their journey over the eastern seaboard of the United States. Only a last-minute tip from Saudi intelligence led to their discovery at air cargo hubs in Germany and the UK. Around five times more PETN had been stuffed inside the cartridges than used in the failed attempt over Detroit on Christmas Day 2009.

"The aircraft could have been brought down," UK Home Secretary Theresa May told the British Parliament days later.

Today, passenger and cargo jets are still vulnerable to a well-disguised bomb containing the colorless and odorless PETN, according to multiple counter-terrorism sources. That is despite improving detection capabilities and greater international cooperation.

"There is no 100 percent, foolproof system for all cargo, all passengers," U.S. Homeland Security Secretary Janet Napolitano told CNN, "but what we can do and are doing is maximizing our ability to prevent such a plot from succeeding. Good intel, good information sharing."

One of those measures is more comprehensive screening. Despite the fact that approximately 60% of all air cargo enters the United States on passenger jets, not all of it is screened.

"Not all cargo is screened; all cargo, however, is looked at in some fashion. So it kind of depends on what you mean. Screening is a term of art, but no, we don't have 100 percent screening of all cargo at all times, that's correct," Napolitano told CNN.

The Department of Homeland Security is also carrying out ongoing assessments of the security of air cargo supply chains, including at points of origin and shippers. In thwarting potential plots, security experts emphasize intelligence tips are the first line of defense.

Despite the discovery of the printer bombs last October, al Qaeda in the Arabian Peninsular (AQAP) was exuberant. "The following phase would be for us to use our connections to mail such packages from countries that are below the radar and to use similar devices on civilian aircrafts in Western countries," the group said in a special issue of its online magazine Inspire.

It also boasted of its technical prowess. "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."

According to explosive detection experts, the X-ray absorption of PETN and ink-toner is indeed similar. "They understood the technology and what it did up to a point," says Kevin Riordan, technical director at Smiths Detection, a UK company that is one of the leading producers of explosive detection.

"They are using novel techniques of concealment, novel materials in terms of containment, and novel materials in terms of explosives," he told CNN in an interview at a Smith facility in Doncaster, England.

The device was so well concealed in the printer that it took UK authorities many hours before they found the explosive device –- even after isolating the consignment containing the suspect package at East Midlands airport, several sources told CNN.

"[It]was examined by specialists and at their first examination they declared it not a bomb," according to Sidney Alford, an explosives expert who helped U.S. authorities in the initial stages of the investigation.

"Only when they received intelligence saying we think it is look again or words to that effect did the awful truth dawn on them. That must really have scared them."

Following the package bomb plot, the Department of Homeland Security mandated that all in-bound air cargo travelling to the United States on passenger jets should be screened by the end of 2011.

But according to explosive detection experts, even 100% screening offers no guarantees.

According to a June 2010 report by the U.S Government Accountability Office (GAO), TSA officials have reservations about the effectiveness of using canine teams to screen for explosives. The white powdery explosive PETN in particular is difficult for sniffer dogs to detect because it has a low vapor pressure, meaning very little of it naturally disperses into the air, according to explosive detection experts.

Screening through physical search is impractical given the volume of cargo and the ease with which PETN can be hidden. Conventional X-ray machines may also fall short, a leading explosive detection expert told CNN.

Explosive detection experts say air cargo departure points around the world need a new generation of equipment. Napolitano says this has become a priority.

"We've now worked with the World Customs Organization, as well as with ICAO [the International Civil Aviation Organization] on improving standards for screening cargo for bombs but also mail. We've also been working on improving technology that gives us better screening capabilities," she told CNN.

The latest generation of explosive detection equipment involves two stages of scanning - an advanced X-Ray machine which provides a high-resolution image of contents which if suspicious prompts a second test. That involves swabbing the outside of the package or analyzing the air inside it. This technique –- known as explosive trace detection - can detect and identify extremely small quantities of explosive residue such as PETN, according to experts.

Smith Detection is working on such equipment. "PETN can be found quite easily in very small amounts using trace detection equipment and in bulk form by X-ray machines," director Kevin Riordan told CNN.

But he conceded that even with the latest equipment, al Qaeda could take steps to make detection more difficult. "We’d have to say there is always a way through," he told CNN. "The risk is never removed totally."

TSA administor John Pistole agrees. "We are in the risk mitigation, risk management business, not risk elimination," he said, "so we have multiple layers of security in place."

Explosive detection experts tell CNN that well-sealed explosives can be more difficult to detect. Ibrahim al Asiri, the Saudi AQAP bomb-maker suspected of being behind the package bomb plot and the Christmas Day 2009 plot which also involved a PETN-based device, appears to have been well aware of this.

"Good packaging and sealing of the explosive material prevents sniffing dogs or equipment from detecting the explosives," the authors of AQAP’s Inspire magazine wrote.

The new generation of machines are now routinely used to scan all checked and hand luggage at airports in the United States and Europe, according to explosive detection experts, but not all air cargo.

According to explosive detection experts, scanning air cargo presents a greater challenge than checked luggage because a high proportion of it has been consolidated into large pallets by the time it has arrived at airports and is ready to be loaded onto planes.

Efforts are therefore underway to scan air cargo as far "upstream" as possible. Large shipping companies have begun integrating the new generation of machines into their automated sorting systems.

Homeland security experts say that the private sector must step up to the plate if air cargo is to be secured.

"The U.S. has polices on how much cargo needs to be screened inbound. We can control that to some degree but we are very much reliant on our partners," Robert Liskouski, former director of infrastructure protection at the Department of Homeland Security, told CNN.

"There is not one unified body in the world that overlooks cargo standards," Liskouski said.

Industry insiders in the field of explosive detection say that some parts of Africa and the Middle East are lagging behind in deploying the latest generation of explosive detection technology. They say more robust internationally agreed standards are vital.

"The global supply chain presents some challenges because the weakest link in that global supply chain can adversely affect the security throughout that supply chain," said Pistole.

"That IED on a plane is what we are focused on because of what we've seen with Christmas Day, obviously with Richard Reid the shoe-bomber, the liquids plot from August of '06, and then the Yemen cargo plot with the explosives on cargo planes in the toner cartridge printers," Pistole said.

To improve security in the shorter term, the United States and international partners have worked together with the governments of "high risk" countries such as Yemen to improve their air cargo detection capabilities. Smiths Detection, for example, has sold hand-held detection devices to international shipping companies operating in Yemen.

Pistole told CNN that all high risk cargo is being screened, including 100% of cargo from Yemen.

CNN asked explosives expert Sidney Alford to demonstrate the explosive power of the printer bombs dispatched by AQAP, by replicating as closely as possible the design and dimensions of the devices they used. The resulting explosion violently twisted and tore apart an aluminum sheet placed under the printer to simulate an aircraft’s skin. "If that had been part of an airplane’s fuselage then heaven help the airplane: it would have been a terminal event I’m afraid," Alford said.

The threat is all the more serious, according to U.S. counter-terrorism officials, because the AQAP bomb-maker responsible for constructing the printer cartridge devices - Ibrahim al Asiri - remains at large. The concern is that he is improving his already significant bomb-making abilities and could still be producing PETN, whose relative stability makes it convenient for transportation across long distances and slow to deteriorate.

"If he is still out there, then the threat is still out there," Alford said. "Whoever designed this, he is at the clever end of the scale."

While airports and couriers around the world were probably keenly aware of printer cartridges, Alford said, there were other ways to hide explosives. "And they’ll be wondering, what else have we been careless about?"