Timeline: Al-Shabaab's deadly mission, bitter rivalries

Assailants stormed a Nairobi shopping mall in September 2013, leaving at least 68 people dead and shocking the world with the brutality of the attack. A Somali militant group called al-Shabaab, which has strong ties to the al-Qaeda global network, claimed responsibility.

Al-Shabaab has been implicated in several attacks in Somalia. But the militants have also vowed to strike at East African countries, such as Kenya and Uganda, that have aided Somalia's fledgling government as it seeks to recover from decades of chaos and civil war.