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.