Any visitor who walked the streets of Paris in the days or weeks following the attacks of January 2015 would definitely have witnessed a particular form of graphic irruption: the dissemination of messages of solidarity and mourning, and the repetition, within this mass of writing, of the formula ‘I am Charlie’. Although the situation was different, the responses to terrorist attacks in January 2015 and the 9/11 aftermath are comparable by the ‘writing event’ (Fraenkel, 2002, 2018) they produced: temporary and atypical dispositifs of writing turned to the public space in order to be read or at least seen by passers-by. This article, structured along chronological lines, traces the evolution of the viral formula over the long term from Twitter to the urban public space. Firstly, the author focuses on the origin and meanings of the statement and formulates several hypotheses that may explain its wide circulation on social networks. Secondly, she analyses the post-attack graffiti based on databases of several private graffiti-cleaning companies in order to highlight the temporary sacralization of illegal writings. The ‘ Je suis Charlie’ phenomenon is interesting in many ways: its staggering, massive diffusion; the apparent unanimity with which it was greeted in the world of politics and the media; and the way it was managed by local authorities.