The current dissertation explores the online mediatization of the Boston Marathon bombing crisis by an American newspaper (The New York Times) and a French one (Le Figaro) and their publics’ reactions to it. The research was conducted along two main analysis axes: (1) the main frames and themes through which the journalists and the publics gave meaning to the event, and (2) the characteristics of the online public spheres therefore created. The comparative perspective on the journalistic frames showed a strong tendency of homogenization, as the same main frames appeared in both analyzed newspapers. However, the online comments analysis revealed that in both cases, while the publics debated the event within those journalistic frames, they also negotiated or assigned new meanings, therefore creating new themes. The comparative perspective on the online public spheres showed that the major normative conditions of a public sphere were achieved. Nevertheless, certain differences were found that could be explained as pertaining to different cultures: the French debates were characterized by more moments of subjective personal involvement and flaming. From a journalistic practice standpoint the findings indicated that both the American and French publics critically scrutinized every piece of news information, addressed precise requests, and expected journalists to reply and fulfill their informational needs.