This chapter analyses the Norwegian authorities’ presence on Twitter during the 22 July 2011 terrorist attacks. Twitter activity by two official institutions is analysed in particular, namely, the blood bank at Oslo University Hospital and the Norwegian Police Security Services (PST). Our findings show that the Norwegian authorities were almost completely absent on Twitter during the critical hours of the terrorist attack, and that there was no coordination and synchronisation of communication from the authorities. This official silence allowed the diffusion of speculation and misinformation to take place; these were neither corrected nor addressed, as the analysed PST case shows. In contrast, the blood bank used Twitter to mobilise blood donors to address an acute problem: a shortage of blood to treat casualties. The chapter concludes by offering recommendations to the authorities for future major incidents.