In late October, two 18-year-old men from the Welsh city of Cardiff were arrested on Kenya's border with war-torn Somalia. The father of one of them told the BBC he believed his son had been brainwashed and was on his way to join an Islamic holy war.

Kenyan authorities quickly sent the two Britons -- one of Somali ancestry, one of south Asian descent -- back to Britain. After questioning by police, they were released without charge.

The arrests, which occurred just as Kenyan security forces launched an air and ground incursion into Somalia, shone a light on an increasing concern for British and U.S. counter-terrorism experts -- the interest of young British Muslims in joining al Shabaab.

The Somali-based Islamic militant group is aligned with al Qaeda and its Yemen-based affiliate, Al Qaeda in the Arabian Peninsula (AQAP).

European officials say a steady stream of British citizens and residents have been making their way to Somalia to join up with bands of al-Shabaab militants who patrol, and in some cases control, patches of Somalia's fragmented territory.

Some of the group's British recruits come from within insular communities of Somali immigrants to Britain, based in working class neighbourhoods of London and a few provincial cities, including Cardiff, a seaport where Somali immigrants involved in the shipping trade established a presence years ago.

But officials say Shabaab has also succeeded in recruiting Britons of much more diverse backgrounds, including British residents of Pakistani ancestry and Muslim converts with ethnic Anglo-Saxon pedigrees. The officials say dozens of English- speaking Westerners have travelled to Somalia to join or train with Shabaab.

British authorities believe the potential problems posed by Britons connecting with al Shabaab could pose threats not just to Somalia and its neighbours - where investigators believe Shabaab is extending its activities - but to Britain as well.

The British authorities are trying to spot and track people who travel to Somalia from Britain and might have come into contact with Shaabab.

REAL THREATS

The problem is not altogether new. John Sawers, head of Britain's Secret Intelligence Service, otherwise known as MI-6, said last year that Al-Qaeda affiliates in Yemen, Somalia and North Africa pose real threats to Britain.

About the same time, Jonathan Evans, head of Britain's domestic Security Service, also known as MI-5, spoke of plots against Britain that were originating in Somalia and nearby Yemen following counter-terrorism successes against al Qaeda's central leadership in Pakistan.

Authorities believe the problem has only continued to grow over the 12 months.

Somalia today resembles 1990s Afghanistan as a potential seedbed for terrorism, Evans said, adding: I am concerned that it is only a matter of time before we see terrorism on our streets inspired by those who are today fighting alongside al Shabaab.

Counter-terrorist capabilities have improved in recent years but there remains a serious risk of a lethal attack taking place, Evans said.

Investigators in Europe and the U.S. point to a recruitment video, circulated via the internet late last year, as evidence of Shabaab's desire to appeal broadly to militants from outside Somalia. The video features shots of fighters from all over the world appealing to their fellow Muslims in various languages, including English, Swedish and Swahili.

In the past, Shabaab had circulated several video messages presented by Omar Hammami, an American from Alabama who converted to Islam, travelled to Somalia and became a prominent spokesman for the group.

Hammami's messages included hip-hop chants taunting the United States to make him a martyr in a drone attack.

The theme of last December's video, by contrast, was a more general appeal to would-be militants to join the fight out of religious devotion. Speaking with a heavy British accent, one of the militants featured in the message, who went by the pseudonym Abu Dujana, made what appeared to be a rallying call to go to Somalia.

He urged all Muslims that are living in the lands of disbelief, the lands of oppression, to (migrate) to the land of glory, to the land of (power) to the land of jihad.

Some of the recruits who travel to Somalia only spend a few months or a year with the militants, then return to Britain and resume normal lives, apparently having purged their aspirations to religious purity.

Officials allege that other British and American recruits are sent by their Shabaab leaders to whatever frontlines of conflict are active at the time, using them as simple foot soldiers in struggles against local antagonists who can range from tribal or rival militant factions to better-organised and U.S.-backed security forces from neighbouring Kenya and Ethiopia.

What most concerns European and U.S. counter-terrorism officials are Shabaab recruits from Britain, the United States or other European countries who return to their home countries after training with militants and then, either on their own or after recruiting small cells of followers, begin to plot attacks.

AMERICAN RECRUITS

Last July, investigators from the Homeland Security committee of the U.S. House of Representatives issued a report which found that more than 40 Americans from Muslim-American communities around America had joined Shabaab since 2007, including two dozen from the midwestern metropolis of Minneapolis alone.

Congressional investigators said that of the 40 alleged U.S. recruits to Shabaab, three returned to the U.S. and were arrested, one is awaiting extradition to the U.S. from the Netherlands, and 15 are believed by U.S. authorities to be dead. In a speech last April, Mark Giuliano, a senior counter-terrorism official with the FBI, said that 12 U.S. citizens had been killed in Somalia fighting for Shabaab since 2006.

By one estimate, the Congressional report said, at least three of the dead American Shabaab recruits had killed themselves in suicide bomb attacks.

But Congressional investigators said they were particularly alarmed by the fact that as many as 21 American Shabaab fighters (were) still at large or unaccounted for.

Moreover, the Congressional report said, at least 20 Canadians of Somali descent had also disappeared and were believed by Canadian authorities to have joined Shabaab.

The committee said that there was considerable legitimate travel by Somali-Americans between the U.S. and east Africa but that due to the anarchic situation in Somalia it was difficult for U.S. agencies to find out what travellers from the U.S. actually did when they got there.

As a consequence, the committee said, some U.S. counter-terrorism officials increasingly fear that they have not identified all the American travellers to Somalia who have come into troubling contact with or joined Shabaab.

A European official familiar with Shabaab activities in Britain said that U.K. authorities believed they knew the identities of many, if not most, of the British citizens and residents who had travelled to Somalia to join the group or train with it.

But the official said authorities could never rule out the possibility that someone from Britain could travel to Somalia, train with Shabaab and then return to Britain, all in secrecy and without coming to the notice of authorities.

Some experts say there are signs Shabaab itself is outgrowing its infatuation with foreign militants. Will Hartley, a terrorism expert at IHS Jane's, said the group had been acting more as an underground terrorist network than a military insurgency.

In this configuration, Hartley said, Shabaab would have less use for non-ethnic Somali foreign recruits. In its guise as an insurgent military force, foreigners were able to play a useful role, contributing military or other skills or simply providing raw manpower, despite the fact that they did not speak Somali and did not look like Somalis, said Hartley.

But now that al Shabaab is apparently seeking to strengthen its clandestine networks in order to carry out more bombings in Mogadishu in particular, the conspicuousness of the foreigners would be a drawback. These people stand out far too much to be able to play a role in covert operations and help form underground cells in the city, Hartley said.

(Additional reporting by William Maclean in London; Editing by Richard Balmforth)

(This story was corrected to make clear one of the two arrested men was of south Asian, not Somali, descent)