On Saturday, a gunman opened fired in a Walmart store in El Paso, killing 22 people and injuring more than two dozen others.

The authorities said the suspect, Patrick Crusius, a 21-year-old white man, wrote a hate-filled, anti-immigrant manifesto that appeared online minutes before the massacre. Echoing the man accused of fatally shooting dozens of people at two mosques in New Zealand in March, the El Paso gunman’s manifesto mentioned the “great replacement,” a conspiracy theory that warns of white genocide.

The man often said to be behind the great replacement theory is Renaud Camus, a French writer who in 2017 was profiled by Thomas Chatterton Williams, a journalist and author who has written extensively about race.

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Even before the rampage in El Paso, Mr. Chatterton Williams argued that the rallying cry of the white nationalists who marched on Charlottesville, Va., in 2017 — “You will not replace us” — could be traced to Mr. Camus and other French theorists like him.