Al Shabaab — aka ’The Youth’ — is an insurgency that has been at war with Somalia’s western-backed government for several years.
One of its most prominent members is Omar Hammami, who is also known as Abu Mansur al-Amriki or The American, and produces pro-jihadist rap music.
Mr Obama met his grandmother during his first visit to Kenya, at the age of 27. He is believed to have visited twice since then. Mrs Obama is thought to have visited the US twice.
She has said that while she is close to the president, they still have to communicate via an interpreter because she speaks no English.
As news of her grandson spread locally in the approach to the 2008 presidential election, a stream of would-be migrants arrived at her door, seeking an American visa to allow them to leave Kenya.
Reporters who visited Mrs Obama in the approach to the election discovered she had a photo of herself and a young Mr Obama on display in her home.
She also had a poster for his first Senatorial election campaign, on which he had written “Habariuku Mama Sarah”, which is Swahili for “Hello Granny Sarah”.
Last week Mohamed Kalaf, a senior Al-Shabaab commander, vowed revenge against the US following the death of bin Laden. “We shall make them regret,” he told a press conference in Mogadishu.