Airport authorities in Dubai rarely screen cargo that transits through the country, making it all but impossible to find bombs like the one intercepted in Dubai on Friday without a tipoff.
When cargo arrives in Dubai, it is set aside for a 24-hour "cooling" period before it continues on to its destination.
"Usually, the transit cargo does not get screened anywhere in the country unless there is a suspicion," said Saif al Suwaidi, director general of the General Civil Aviation Authority.
In London, where the second bomb was found on a UPS plane, a greater emphasis on cargo screening is overdue, the British Airline Pilots' Association said yesterday. The focus on security screening of passengers had "left the door open" for cargo flights to be targeted, it said. The association said it had been warning for years of the vulnerability of freight shipments to terrorism.
Jim McAuslan, the association's general secretary, said: "It is good news that the combined intelligence services have worked together effectively to foil the plot. But, as the eyes and ears on the front line, pilots' warnings about the whole security regime now need to be listened to."
In the UAE, Mr al Suwaidi pointed out that a huge amount of cargo goes through Dubai and not much of it is screened. "On normal days, they check one to two a day - a very low percentage of the whole amount."
The duty to inspect, he said, lies with the country of origin - in this case, Yemen.
Warning of an explosive first came from Saudi intelligence officials, who passed their information on to the Americans. Dubai was alerted and investigators here found the bomb in a FedEx cargo plane.
"Even if you have a very good screening system, it won't detect everything," Mr al Suwaidi said. "The key is to have very good intelligence, and exchange of intelligence between countries. That is what we are trying to do now."
He and several other Arab leaders pledged yesterday at the Doha Aviation Summit to share information "at the highest level". Some 200 delegates from 35 countries attended the conference, which had been scheduled prior to the discovery of the bombs.
"Yemen will have all the support of the Arab world," said Abdul Aziz al Noaimi, chairman of the Qatar Civil Aviation Authority, speaking on behalf of the Arab Civil Aviation Commission. "We have the expertise to help them because the threat is to all countries."
As a precaution, the shipping companies FedEx, UPS and TNT said they would halt imports from Yemen. "This is in agreement with the American government and for our own peace of mind," said Mark Woodcock, a spokesman for TNT.
The United States Postal Service and its counterparts in Britain and France said they had suspended mail and air freight from Yemen.
Commercial airlines such as Qatar Airways and Emirates regularly ferry parcels on passenger flights on behalf of shipping companies. Qatar Airways said it would continue its regular operations to Yemen, including its weekly Monday cargo flight.
Emirates said a recent flight to the US had not included any packages from Yemen, but declined to say whether it had imposed a broader ban.
Mr Suwaidi said stopping shipments was not the answer. Nor will increased screening solve the problem, said Mustafa Alani, a security analyst at the Dubai-based Gulf Research Centre.
The plotters disguised their explosives so well that not even a human eye - let alone an x-ray machine - would have detected it, he said. The Dubai bomb contained parts normally found in a printer, he noted.
"It is very difficult to spot electric equipment or powder in a machine which already has an electric system and powder," he said.
Adding to the difficulty, he said, bomb-makers are growing in sophistication. While they continue to use PETN - a white, odourless powder - they are also choosing more subtle detonators.
In 2001, the "shoe bomber" Richard Reid was thwarted on a France-US flight while trying to light matches to ignite explosive. Last Christmas, the "underwear bomber" Umar Farouq Abdul Mutallab tried to create an explosion en route to the US by injecting acid with a syringe.
The detonator this time - an electronic system - was the smartest yet, said Mr Alani. "It was very clever."
However, a security source in London saw nothing clever about the addresses on the packages - synagogues in the Chicago area.
"If you are an Arab and you want to get a bomb on a flight to the US without sounding alarm bells, the last thing you would think you would do would be to address it to a Jewish organisation," the source said.
He said he was amazed that packages with those addresses could have been dropped off in Sana'a and not be screened.
Amid the rush by governments to improve security of freight, there were warnings that it simply would not be feasible to check every item among the millions sent daily around the globe.
Lord (Alex) Carlile, the British government's independent reviewer of terror legislation, told the BBC yesterday: "To search every parcel is a physical impossibility. The technology is good but we can't rely on technology alone.