PARIS, FRANCE - NOVEMBER 15: A Police officer stands in front of Notre Dame Cathedral ahead of a ceremony to the victims of the friday's terrorist attacks on November 15, 2015 in Paris, France. As France observes three days of national mourning members of the public continue to pay tribute to the victims of Friday's deadly attacks. A special service for the families of the victims and survivors is to be held at Paris' (Photo by Pascal Le Segretain/Getty Images)