Dozens of journalists, media outlets and politicians, from Louisiana to New York City, found their Twitter accounts inundated with messages about the disaster. “Heather, I’m sure that the explosion at the #ColumbianChemicals is really dangerous. Louisiana is really screwed now,” a user named @EricTraPPP tweeted at the New Orleans Times-Picayune reporter Heather Nolan. Another posted a screenshot of CNN’s home page, showing that the story had already made national news. ISIS had claimed credit for the attack, according to one YouTube video; in it, a man showed his TV screen, tuned to an Arabic news channel, on which masked ISIS fighters delivered a speech next to looping footage of an explosion. A woman named Anna McClaren (@zpokodon9) tweeted at Karl Rove: “Karl, Is this really ISIS who is responsible for #ColumbianChemicals? Tell @Obama that we should bomb Iraq!” But anyone who took the trouble to check CNN.com would have found no news of a spectacular Sept. 11 attack by ISIS. It was all fake: the screenshot, the videos, the photographs.