"It appears that he joined an affiliate of al-Qaeda and that this group, al-Qaeda in the Arabian Peninsula trained him, equipped him with those explosives and directed him to attack that plane headed for America."
US military aid to Yemen has been inconsistent in recent years, with Sana'a receiving $4.3m in 2006, up to $26m in 2007, down to nothing in 2008, and back up still higher, to $67m in 2009.
Mahan Abedin, director of the Center for the Study of Terrorism, says that while US aid has fluctuated, the al-Qaeda presence in Yemen has not.
"They've had Yemen in their sights for a very long time ... at least as long as September 11. Even prior to that … Yemen has had a strong militancy problem since the late 1980's."
The fear among Western governments is that Yemen is poised to collapse amid extreme poverty and dwindling resources, and the ungoverned state would provide a safe haven for armed groups.
Gordon Brown, the British prime minister, has said he will hold a meeting in London on January 28 to discuss how to counter radicalisation in Yemen.
The government in Sana'a is struggling to contain an uprising by Shia Houthi rebels in the country's north and a secessionist movement in the south, in addition to the re-grouping of al-Qaeda fighters in recent years.
General David Petraeus, the commander of the US central command, visited Yemen on Saturday to discuss military and economic co-operation with Ali Abdullah Saleh, the Yemeni president, according to officials from both countries.
Petraeus had announced a day earlier that US military aid to Yemen "will more than double this coming year".
Experts say Obama's military options to combat al-Qaeda in the Arabian Peninsula are limited.
David Newton, a former US ambassador to Yemen, said his main option "is one he's already doing - using predators [drones] to go after terrorists that they are able to locate".
"To do that, [he] has to co-operate with Saudi security and Yemeni security, and [use] our own independent means to find them."
Newton said the greater challenge will be to stabilise the country, which has seen a failing economy, severe water shortages, and a ballooning population in recent years.
Other al-Qaeda affiliated groups, such as in Somalia, have meanwhile vowed to support the Yemeni fighters.
Al-Shabab, the leading anti-government armed group in Somalia, said on Friday that it was ready to send reinforcement to al-Qaeda in the Arabian Peninsula should the US carry out retaliatory strikes, and urged other Muslims to follow suit.
Abedin said numerous fighters from Africa may have already arrived in the country.
"One of the reasons for the current concerns is that hard intelligence has come in that an assortment of militants, especially from eastern Africa, have somehow managed to get into Yemen," he told Al Jazeera.
"There's clearly some kind of smuggling route that [transports] these elements from countries in Eastern Africa, including Nigeria, to Yemen where they receive training in remote areas."
British officials said they would push for a larger UN peacekeeping force to be deployed to Somalia, to try to contain the situation in the failed state.
The prime minister's office also said the US and UK had agreed to increase funding for Yemen's coast guard, in an effort to combat piracy in the Gulf of Aden, although Washington has not yet confirmed the move.
Pirates operating in the waters between Somalia and Yemen have captured at least four ships in the last week.