Feds are tracking Americans' social media to identify dangerous conspiracies. Critics worry for civil liberties.

Since 2018, at least three white supremacist mass shooters in America have referred to the “Great Replacement” theory, a conspiracy claim that white people are being replaced in the U.S. and elsewhere by immigrants of color.

That’s the narrative Robert Bowers alluded to on social media before killing 11 people at a synagogue in Pittsburgh in 2018. Patrick Crusius, who killed 22 people at a Walmart in El Paso, Texas, less than a year later, was inspired by the racist trope. So was John Earnest, who killed one person...