Yemen provides a potentially easy entry point for foreign radicals to link up with al Qaeda, with a number of popular Islamic religious and Arabic-language schools that attract students from around the world.
Already there has been at least one confirmed case - the young Nigerian student allegedly used a San'a language school as a cover to enter the country and meet with al Qaeda militants for training, before he made a botched attempt to blow up an American passenger jet on Christmas Day.
Since then, Yemeni security forces have cracked down, arresting a dozen Americans and an assortment of Europeans on suspicion of contacts with al Qaeda.
Evidence that those arrested actually contacted al Qaeda is sketchy, and some were likely caught up in the intensified Yemeni search. Two of the arrested Americans have since been deported and an unspecified number have been released, Yemeni security officials said, speaking on condition of anonymity because they were not authorized to talk about the cases.
But concern is high over the potential for al Qaeda's affiliate in this country to recruit militants with American or European passports. Among the senior figures in the group is the U.S.-born radical cleric , whose English-language sermons advocating jihad, or holy war, against the United States have inspired a number of Western-born militants.
Al-Awlaki, who the U.S. has put on a list of militants to kill or capture, was in e-mail contact with the Army psychiatrist accused of last year's deadly shooting spree at the Fort Hood, Texas military base. U.S. investigators say he also helped prepare Abdulmutallab for his failed attempt to bomb the Detroit-bound airliner.
The group has also . In its second issue, posted in October, a young American militant, Samir Khan - believed to be the magazine's producer - boasted how he had moved to Yemen from his home in North Carolina and joined al Qaeda's fighters, pledging to "wage jihad for the rest of our lives."
Another American, Sharif Mobley - a 26-year-old from New Jersey of Somali descent - went on trial last week for killing a Yemeni soldier during an escape attempt in March. Mobley had been arrested originally for suspected links to al Qaeda, and while being treated in a hospital, he reportedly convinced a guard to unshackle him, then grabbed a guard's gun and opened fire.
In June, a German national was said by the government to be among four detained in connection with a failed suicide bombing targeting the British ambassador two months earlier.
So far, Yemeni and U.S. investigators have not said whether Western-born militants in Yemen were directly involved in plotting or carrying out the latest plot, in which two explosive devices hidden in packages addressed to Chicago-area synagogues were mailed from Yemen and intercepted Friday on planes transiting through Dubai and Britain.
But CBS News homeland security correspondent Bob Orr reports that U.S. officials now have no doubt the Yemen-based al Qaeda franchise was behind the cargo bomb plot.
Sources tell CBS News that about 15 other "suspect" packages sent from Yemen around the same time as the bombs have now been located and cleared. But, Orr reports, officials have voiced concern that there could still be other explosive packages out there, and either way, AQAP presents a long term threat to the U.S.
John Pistole, the head of the Transportation Security Administration, told CBS' "The Early Show" on Monday that the TSA was working with its partners across the globe to try to identify where any other suspect packages might be.
"We're aware that were facing a determined enemy," Pistole told "Early Show" co-anchor Maggie Rodriguez, adding that TSA agents had been sent to Yemen to help screen U.S.-bound cargo - so when a ban imposed Saturday on all shipments from the Arab country to the U.S. is dropped, packages can be shipped more safely.
According to CBS News terrorism consultant Juan Zarate, al Qaeda's affiliate group in Yemen is "the most dangerous of al Qaeda's branches or allies."
Yemeni authorities hunted suspects linked to the group, but computer engineering student arrested Saturday, saying someone else had posed as her in signing the shipping documents.
Yemen: The Next Front Line Against al Qaeda
Al Qaeda in Yemen is widely thought to have some 300 core fighters, most of them Yemenis and Saudis.
Recruiting militants living in the West would offer the group wide possibilities to carry out attacks. Radicals with European or North American passports can travel freely across most of the world, while militants from places like Saudi Arabia, Egypt or Pakistan must go through the thorough background checks involved in visa applications.
The large numbers of foreign students who have long flocked to Yemeni language and religious school could provide a pool to draw from. Students with militant tendencies can easily fall off the radar and contact al Qaeda elements in a country where the central government is weak and the police are notoriously corrupt.
One popular destination is the Dammaj Institute, a school for Islamic religious studies in the northern town of Saadah run by adherents of the ultra-conservative Salafi ideology of Islam shared by many in al Qaeda. Dammaj has some 700 students from around the world.
Another is Al-Iman University in the capital, San'a, run by hard-line cleric Abdul-Majid al-Zindani, who is considered by Washington a "specially designated global terrorist" but is also close to President Ali Abdullah Saleh. His university teaches Islamic studies with a clear militant slant. Among its most notorious alumni is John Walker Lindh, a U.S.-born Taliban fighter now serving a 20-year sentence in the United States.
But Abdulmuttalab's case showed how even the mainly secular language schools in San'a can be a gateway.
Foreign students, including young Europeans and Americans, are attracted to San'a for study because of its exotic lifestyle, unique architecture and because it is relatively cheap to live in. Students mostly live in the historic old city in the midst of poor Yemenis.
"Yemen is attractive to devout foreign Muslims because of its conservative nature and because clerics wield significant influence," said political analyst Mohammed al-Sabry. "It is the closest you can get to a typical Islamic society."
Abdulmutallab may have single-handedly destroyed the roaring Arabic-language school business in San'a. After his failed attack, Yemeni authorities clamped down, introducing a visa regime for foreign visitors and running stringent security checks on foreign students. The Arabic schools that boasted up to 100 students a year ago now have 15 and most of their classrooms sit idle.
"No new students are coming," said Abdul-Rab Mohammed, an administrator at the Sana'a Institute for Arabic, where the Nigerian was enrolled. "Students have been kept away by the visa requirements and security checks."
The school now has only six students, including two Americans - a young couple from Pennsylvania.