Within days, the two packages had advanced through four countries in at least four different airplanes — two of them carrying passengers — before they were identified in Britain and Dubai after an 11th-hour tip from Saudi Arabia's intelligence service set off an international terrorism alert and a frantic hunt.
The foiling of the package plot was a significant success in an era of well-publicized intelligence breakdowns and miscommunications.
It was also a sobering reminder to officials around the world that quick response to timely intelligence rules the day. Despite the billions of dollars governments have spent on elaborate airport technology to guard against terrorism threats, the packages would probably have been loaded onto planes bound for the United States, but for the Saudi tip.
But the plot also points up holes in the system, particularly the security of cargo flights, that have already caused criticism abroad and are likely to rekindle new debates in the United States.
In Qatar, officials acknowledged Sunday that one of the packages had been carried on two Qatar Airways passenger planes, apparently having eluded the airline's cargo screening system. In Britain, officials were embarrassed about how long it took the authorities to identify one of the packages as a carefully concealed bomb.
American and Yemeni officials still have little hard evidence about who was involved in the thwarted attack. On Sunday officials in Yemen discovered that Ms. Samawi's identity had apparently been stolen, and that she was not the same woman who dropped off the packages. Ms. Samawi was released on bail on Sunday, and the authorities in Yemen have thus far arrested no other suspects.
It was one more piece of a carefully designed and cleverly disguised plot that investigators believe was conceived by Al Qaeda's affiliate in Yemen, the group that American officials say might pose the most immediate threat to American soil.
In television appearances on Sunday, John O. Brennan, President Obama's top counterterrorism adviser, said that American and British authorities were leaning toward the conclusion that the packages were meant to detonate in midair, en route to their destinations in Chicago. If that turns out to be the case, it would be a rare attack aimed at the air cargo system — one of the foundations of the global economy — rather than the passenger system, which has received the most attention from governments working to avoid a repeat of the Sept. 11, 2001, attacks.
For the most part, governments around the world had bet that it was less likely that the cargo system would be the target of attacks, given that its flights carry few passengers.
“It is time for the shipping industry and the business community to accept the reality that more needs to be done to secure cargo planes so that they cannot be turned into a delivery systems for bombs targeting our country,” Representative Ed Markey, Democrat of Massachusetts, said in a statement.
Congress in 2007, in legislation proposed by Mr. Markey, mandated that all air cargo be inspected before it is loaded onto passenger planes, setting an August 2010 deadline for the requirement. But as of the deadline, only about 65 percent of the cargo headed to the United States on passenger planes from abroad is inspected — and a far smaller proportion coming to the United States on all-cargo flights is physically checked, as these planes are not subject to the mandate.
Even when the cargo is checked, air carriers in certain countries use equipment like X-ray detection devices or a visual check by an airport worker that often cannot identify packages with bombs, because the small amount of explosive material can be carefully hidden inside a routine electronic device, like a computer printer.
Interviews in Washington, London and the Middle East reveal how the two bombs made their way through several countries before the tip from Saudi intelligence officials caused them to be pulled from airplanes.
The bomb dropped at the U.P.S. office in Sana ended up in East Midlands Airport, near Nottingham, England, by way of Cologne, Germany. A terrorism alert from Washington provoked a search for the package, which was found and kept from being shipped to the United States. But British authorities took more than 20 hours to determine that it contained hidden explosives.
Theresa May, the British home secretary, told the BBC that the government would review its security arrangements for handling air cargo entering or passing through Britain in the wake of the printer-bomb plot, but declined to give any details.
In Britain, cargo operators are vetted and named “trusted carriers.” Cargo itself is not screened, which some experts said made British airports vulnerable to terrorist exploitation. Ms. May said that any changes would have to take into account economic concerns. “We're well aware of the economic aspects of air freight transport,” she said.
The second package — a bomb hidden inside a Hewlett-Packard desktop printer — was sent out Thursday on a Qatar Airways passenger flight to Doha, the Qatari capital. There it sat for a day, and was then flown 235 miles east to Dubai, in the United Arab Emirates, where it arrived Friday in the local FedEx distribution warehouse.
By that time Emirati authorities had received a warning call from Britain about a suspicious package there, and they identified the printer almost immediately, according to an official familiar with the investigation. Investigators removed and dismantled the explosive, which had been placed into the toner cartridge printer so carefully that all the printer's components appeared to be in place and it might well have passed unnoticed.
A cellphone was concealed in the bottom of the printer, and the printer head was designed to detonate the explosives.
On Sunday, officials in Qatar said in a statement that “the explosives discovered were of a sophisticated nature whereby they could not be detected by X-ray screening or trained sniffer dogs.”
As for who was behind the plot, evidence remains elusive, though officials believe the bombs bear the hallmarks of Al Qaeda in Yemen's top bomb maker. On Friday, the Department of Homeland Security issued a cable saying that the packages might have been linked to two schools in Yemen. If true, that would suggest that foreign students might have been involved in the plot, as in the attempted bombing of a commercial jetliner in Detroit last Dec. 25 by a Nigerian trained in Yemen.
But American and Yemeni investigators are trying to determine whether the schools — listed as the Yemen-American Institute for Language-Computer Management and the American Center for Training and Development — even exist. There is a school in Sana called the Yemen American Language Institute, but it is sponsored by the United States State Department. Its director, Aziz al-Hadi, said in a telephone interview that the school “has never used FedEx or U.P.S.” and did not help foreigners obtain visas. The school does not have a reputation for attracting religiously conservative students, unlike some other language schools in Yemen. There is an American Center for Training and Development in Egypt, but not in Yemen.
Ms. Samawi was released partly because the shipping agent for the courier company was brought into her interrogation and told investigators that she was not the person who had signed the shipping manifest, said a Yemeni official, who spoke on condition of anonymity. The Yemeni authorities have concluded that the plotters deliberately used Ms. Samawi's name, address and telephone number to make the shipment look legitimate. Ms. Samawi's mother was detained Saturday as well, but family friends said that was only because she insisted on accompanying her daughter.
“She is a very open-minded person; we cannot believe these accusations at all,” said Siham Ahmad Haza, 24, who described herself as a close friend of Ms. Samawi's, and a fellow student of computer engineering at Sana University. “She listens to music a lot, especially Western music. She loves foreigners, she's a balanced person.”
Ms. Samawi has two younger sisters, and her father works as an engineer in the Ministry of Agriculture and Water, according to family friends. The family lives in Shamlaan, on the outskirts of Sana.
About 100 students protested at Sana University on Sunday, chanting “Freedom, freedom for Hanan!”
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