During the Westgate terrorist attack of 2013, al-Shabaab used Twitter to claim responsibility for and live tweet throughout the attack. This article analyzes 556 of these tweets to understand the motivations for using Twitter during ongoing terrorist operations and builds up a picture of how al-Shabaab interacted with Twitter throughout the Westgate attack. Conclusions arising from the analysis include that al-Shabaab were primarily concerned with controlling the narrative of the attack and retaining an audience. In addition, the tweets were aimed at a specific geographical audience, indicating that the Westgate attack was primarily motivated by territorial concerns.