In the second installment of J.K. Rowling's North American–wizardry history lesson, now live on Pottermore, she delves into the most well-known tragedy in American witchcraft: Salem. After settling in the New World and living among Native American witches and wizards, immigrants who practiced magic were quickly persecuted by their non-magically blessed peers, the Puritans. Enter the Scourers, an immoral group of wizards who, without any kind of Ministry of Magic to regulate magic in America yet, took it upon themselves to govern the small wizarding population however they saw fit. "Far away from the jurisdiction of their native magical governments, many indulged a love of authority and cruelty unjustified by their mission," Rowling writes. "Such Scourers enjoyed bloodshed and torture, and even went so far as trafficking their fellow wizards."
On their power trip, at least two Scourers, Rowling says, were partly responsible for the Salem Witch Trials, which massacred a number of "utterly innocent" witches, as well as some non-magical settlers. Salem, Rowling reveals, altered the course of history for American wizardry. It caused a mass exodus of witches and wizards from America and scared most pure-blood families still overseas from emigrating to the New World. For that reason, the majority of subsequent American wizards were Muggle-born, which prevented pure-blood elitism from taking over in North America the way it did in Europe (later perpetuated by Voldemort and his cult of Death Eaters).
The wizards who did stay established the Magical Congress of the United States of America (MACUSA) in 1693, whose first order of business was executing any Scourer "convicted of murder, of wizard-trafficking, torture and all other manners of cruelty." But some Scourers escaped and infiltrated the non-magical community, eventually starting non-magical families (any wizard children were "winnowed out"). Secretly embedded in the Muggle world, they instilled in future generations of North American No-Majs, a belief that magic and wizards were real and "ought to be exterminated." So the next time someone tries to shame your Harry Potter obsession, assume the Scourers brainwashed their family centuries ago.