The names of almost 50 people from Britain who have links to al-Shabab – and related organisations – have been established by the BBC.
Most of those on the list travelled to Somalia to fight or attempted to do so.
Somali-based militants al-Shabab carried out the deadly attack on the Westgate mall in neighbouring Kenya.
Lawyers for the family of one British recruit known to have died in Somalia are investigating whether he was killed in a raid involving western military.
Al-Shabab is fighting to impose a brutal version of Islamic law on the country and the group is linked to al-Qaeda.
The 47 names compiled by the BBC are based on a combination of sources, including public records from courts in the UK and abroad, and further first-hand research and accounts.
It is not a complete list of people from the UK who are suspected of having gone to fight because it is impossible to establish or estimate with any certainty how many Westerners have taken up arms in Somalia.
The research found that security chiefs believe at least 32 of the identifiable individuals went to fight.
A further seven were named in British court cases as having attempted to enter Somalia, but their plans were discovered and disrupted.
Others have been accused or convicted of raising money for al-Shabab, or facilitating the travel of others.
Seven people on the list returned to the UK – but the fate of most of the others is completely unknown.
Four of the men on the list are known to be dead, including men who featured on a video released by the group a week ago.
The video included a message from a young man named as “Talha”, who asked Muslims in Tower Hamlets – a borough in east London – to join the jihad in Somalia.
Talha, whose real name was Taufail Ahmed, died in Somalia in November 2012.
The Foreign Office said it had confirmed the death to his family, who are of Bangladeshi origin.
He grew up in Stepney Green in east London. The BBC has tracked down people who knew him at school.
They say he fell into a life of gang crime before leaving that world behind in favour of an extreme interpretation of Islam, before disappearing from the UK.
“For British Muslims he’s doing a disservice,” said one former schoolmate. “For Muslims around the world, he is doing a disservice.”
In the al-Shabab video, the narrator says that “Talha” died after an attack involving American and British forces.
The government has not commented on that claim but his family has asked the legal charity Reprieve to investigate.
“The family were notified by British police officers. The FCO [Foreign Office] have knowledge of the incident and have told us that Taufail Ahmed received a Muslim burial,” said Kat Craig from Reprieve.
“We have reason to believe that there are Somali eyewitnesses who identified those involved as British personnel and for those reasons we believe that further inquiries need to be made and that the family has quite legitimate questions that need to be answered.”