This morning’s Observer column:

The prevalence of conspiracy theories online explains why they tend to crop up whenever we track the cognitive path of someone who, like the alleged Pittsburgh killer, commits or attempts to commit an atrocity. A case in point is Dylann Roof, a South Carolina teenager who one day came across the term “black on white crime” on Wikipedia, entered that phrase into Google and wound up at a deeply racist website inviting him to wake up to a “reality” that he had never considered, from which it was but a short step into a vortex of conspiracy theories portraying white people as victims. On 17 June 2015, Roof joined a group of African American churchgoers in Charleston, South Carolina, before opening fire on them, killing nine.

We find a similar sequence in the case of Cesar Sayoc, the man accused of sending mail bombs to prominent Democrats. Until 2016, his Facebook postings looked innocuous: decadent meals, gym workouts, scantily clad women and sports games – what the New York Times described as “the stereotypical trappings of middle-age masculinity”.

But then something changed. He opened a Twitter account posting links to fabricated rightwing stories and attacking Hillary Clinton. And his Facebook posts began to overflow with pro-Trump images, news stories about Muslims and Isis, ludicrous conspiracy theories and clips from Fox News…

Read on