On the morning of April 15, 2013, an Improvised Explosive Device (IED) was detonated near the finish line of the Boston Marathon, resulting in a large number of casualties. This generated a week-long response under the US National Incident Management System. In this paper, we examine online, terse messages broadcast by responding organizations and their amplification by other official entities via retransmission. Content analysis of official messages shows strong similarities with posting patterns previously observed in response to natural hazards, with the primary exception of themes related to the criminal investigation, suggesting a possible revision of guidelines for public information in light of the needs arising from extended counterterrorism operations undertaken in an urban environment. Network analysis demonstrates message posting and amplification were dominated by local actors, underscoring the importance of local readiness for management of official public information activities in the context of extremely high-profile events.