The call came into the London Metropolitan Police bomb squad in the early hours of the morning. Isolated at the East Midlands airport in central England was a UPS package dispatched from Yemen, containing a laser printer that Saudi intelligence believed had been converted into a bomb.

Before dawn a bomb squad arrived on the scene. The plane had been cleared and left at 4:20 a.m., without the package identified by its waybill number as the laser printer. Officers inspected the printer and lifted out the ink cartridge but found no explosive device. According to security sources, they also brought in specially trained dogs and passed the printer through an X-ray scanner, but those, too, failed to locate any explosives.

The security cordon around the area where the laser printer had been isolated was lifted. But Saudi counter-terrorism officials implored British authorities to re-examine the printer. When they did, they found 400 grams of the high-explosive PETN inside the ink cartridge.

The bomb had been timed to explode hours earlier. But the bomb squad had inadvertently defused the device earlier when they had lifted the printer cartridge out of the printer, disconnecting the explosives from the timer.

A similar drama had been playing out at an airport in Dubai in the United Arab Emirates, where another printer bomb had been located that same day. These were some of the most sophisticated explosive devices ever seen from al-Qaida.

These discoveries on Oct. 29, 2010, sent shock waves through Western capitals. Not only had these bombs gone through screenings at several airports without being detected, they also had traveled on passenger jets during the first legs of their journeys.

And most disturbing of all: For many hours, the explosives went undetected by bomb experts in two countries, despite being right in front of them.

A few weeks after the incident, U.S. Senator 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.

The group that claimed responsibility for the plot -- the Yemen-based al-Qaida in the Arabian Peninsula -- appeared to have found the Achilles heel of international aviation.

While much airport security is concentrated on screening passengers and their checked bags, about half the hold on a typical passenger flight is filled with cargo. In fact, over a third of cargo by volume that entered the United States in 2010 was shipped on passenger jets, according to the Department of Transportation. That is 3.7 billion tons. Another 7.2 billion tons of air cargo came in on all-cargo aircraft, according to the DOT.

And the screening requirements for such cargo are not as strict as they are for passengers and their checked bags.

If it took authorities in Britain and Dubai hours to identify a bomb that was right in front of them, what are the chances of finding such devices amid the millions of tons of air cargo flying into the United States each day?

A difficult quandary

U.S. authorities were already aware of the potential for terrorists to take advantage of lax cargo security. A law that required screening for all cargo on domestic and inbound international passenger flights had taken effect two months before the printer bomb scare.

While the Transportation Security Administration was able to ensure the screening of all domestic cargo, it fell short when it came to screening all inbound international cargo, according to the U.S. Government Accountability Office.

So the TSA announced that the 100 percent requirement would be brought into effect for inbound flights by January 2012. Now, the TSA has indefinitely deferred this goal in favor of a risk-based approach, according to Massachusetts Rep. Ed Markey.

Following the 2010 bomb plot, the United States and its international partners took a number of steps to bolster air cargo security. They banned cargo shipments assessed as too high a risk that originated from or transited through Yemen and Somalia. U.S. authorities implemented enhanced screening for passenger jet cargo assessed as having an elevated risk and tightened procedures for incoming mail. Those requirements have not been made public. The Department of Homeland Security brought in enhanced screening for U.S.-bound shipments on all-cargo aircraft.

While industry insiders say progress has been made, some lawmakers on Capitol Hill express concern about any approach that doesn't involve the screening of all cargo.

"The low-risk cargo does not receive anywhere near the level of security as the high-risk cargo," said Markey, who co-authored legislation mandating screening on passenger jets by August 2010.

"There is no such thing as low-risk cargo because, in the hands of al-Qaida, that cargo becomes high risk."

But some of those on the frontlines of air cargo security point out that the risk-based approach stems from on-the-ground realities.

"Identifying high-risk cargo wherever it is in the supply chain and singling it out for physical screening is the better approach to securing cargo on an international scale," said David Brooks, the head of American Airlines air cargo.

And the industry says TSA mandates are not easy to enforce when they involve other countries that may face logistical challenges in conforming to U.S. inspection standards. Economic factors also played a role in the U.S government's delay in imposing 100 percent inbound screening.

It was a quandary that al-Qaida exploited. "(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-Qaida in the Arabian Peninsula announced after the package bomb plot.