Its goal: sow division in the border regions of Kenya and Somalia, where many of the people are ethnically Somali, analysts say.
Among Al-Shabaab's most brutal acts was the raid on Garissa University College in April that left nearly 150 people dead. Witnesses described how gunmen asked students to recite verses from the Quran. If they couldn't, they were killed.
The group regularly storms buses, particularly this time of the year -- one of the busiest travel seasons in the nation. Throngs make their way to relatives' homes for the holidays, with buses and other public transportation packed.