The results of the public opinion poll performed in January 2015, just after the terrorist attack on the French satirical weekly magazine Charlie Hebdo and the kosher supermarket in Paris, when 17 people were killed, showed that a significant number of French citizens held conspiratorial beliefs about it (17 %). This gave reason to an alternative analysis of public opinion, presented in this paper. We collected 990 on-line articles mentioning Charlie Hebdo from Le Monde web site (one of the leading French news agencies), and looked at the ones that contained words related with conspiracy (in French: `complot', `conspiration' or `conjuration'). Then we analyzed the readers response, performing a semantic analysis of the 16490 comments posted on-line as reaction to the above articles. We identified 2 attempts to launch a conspiratorial rumour. A more recent Le Monde article, which reflects on those early conspiratorial attempts from a rational perspective, and the commentary thereon, showed that the readers have more interest in understanding the possible causes for the onset of conspiratorial beliefs then to delve into the arguments that the conspiracists previously brought up to the public. We discuss the results of the above semantic analysis and give interpretation of the opinion dynamics measured in the data.