http://heavy.com/news/2016/07/bastille-days-terrorist-attack-nice-france-photos-videos-watch-see-promenade-des-anglais-isis/
Tweets, pictures, ISIS posted pictures already on their site.
There is also the fact that it is very efficient to claim responsibility for what is called a terrorist attack even if it isn’t or your group didn’t do it, in order to build up your street cred.