From ArtForum:
In the days and weeks following the terrorist attacks in Paris on November 13, 2015, archivists working for the mayor’s office collected some 7,709 letters, poems, photos, and drawings that were left on the sidewalks in front of sites of the shootings and around the Place de la République.
Read the Complete ArtForum Article
Direct to Digital Archive (in French)
Documents can be searched by formats, large or small, sites (Bataclan, the Belle team, etc) or date of collection.
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Source Article in French (via BFMTV) ||| English Translation (via Google) ||| English Translation (via Bing)