French provocateur Renaud Camus' 2012 book The Great Replacement warns of a doomsday scenario in which Europe is overrun by nonwhite, Muslim immigrants from North Africa and sub-Saharan Africa.
In the years since its publication, the concept has spread from France to become a right-wing article of faith across Europe.
In 2018, Horst Seehofer, Germany's newly appointed interior minister, declared that "Islam does not belong to Germany" months before engineering a failed standoff against Chancellor Angela Merkel over migration.
In Italy, the far-right politician Matteo Salvini campaigned for office - and won - by promising to close mosques and stop the migrant "invasion".
Hungary's Viktor Orban has decried European Union member states "where a mixed civilisation is already an inevitability," saying they present a threat to "the Christian identity of Europe".
On Monday, Marine Le Pen, the leader of France's far-right, denied she had ever alluded to the "great replacement" theory, despite her regular recycling of anxieties about Islam and immigration.
"I have never used that term," she told France 3 Television, adding that she was not even aware what it meant - a claim demonstrably untrue.
Although Le Pen has kept her distance from Renaud Camus and dismissed his signature idea, she showed her familiarity with it in a 2014 interview. "The concept of the 'great replacement' requires an established plan," she told France's Journal du Dimanche newspaper. "I don't participate in this conspiracist vision."
Le Pen's father and niece, however, have cited the great replacement theory many times, and favourably.
To many French Muslims, the Christchurch massacre was a tragic extension of normalised hatred that has also inspired anti-Muslim and anti-immigrant attacks - some of them foiled - in France and elsewhere in Europe.
The connection between right-wing xenophobia and violence can be clear, as in the 2018 attack in Macerata, Italy, in which Luca Traini, a failed candidate for local office from Salvini's own party, wounded six African migrants in a shooting rampage. Salvini, now Italy's deputy Premier, condemned the Christchurch shooting but added that "the only extremism deserving attention is the Islamic one".
"What the terrorist [suspect] saw when he came to France to visit," said Marwan Muhammad, a Muslim community organiser in Paris, "is the political narrative which makes him interpret the presence of Arabs and Muslims and blacks and Roma not as a sign of diversity, but as a sign of 'invasion.' "
For Jean-Yves Camus, a political scientist and scholar of the French far-right, who is unrelated to Renaud Camus, there is no empirical evidence connecting the Christchurch shooter to contemporary French discourse, for now. What is striking, he said, is that few seem interested in finding any.
"What I'm waiting for is the real inquiry in how and when the terrorist came into France," Camus said. "Did he meet with any right-wing group or individual? How long did he stay and where did he go?"
"We focus so much on the Islamist terrorist movement," he added, "that when it comes to an extreme-right terrorist, we simply do not believe that it's as much of a threat."
For the moment, most is known about Tarrant's itinerary in Eastern Europe, which reflects an interest in the conception of a white, Christian, embattled Europe. According to Bulgarian officials, who have investigated the Australian's travels in the region, he visited several Balkan countries in late December 2016 - including Bosnia, Croatia, Serbia and Montenegro - and Bulgaria in November 2018. Travel records show he rented a car for a week to visit sites in Bulgaria connected to Christian Orthodox battles against the Ottoman Turks.
One hotel manager, who told Bulgarian media that he had noticed a Pakistani stamp in the suspect's passport, said the Australian stayed a night in Pleven, a town in northern Bulgaria known for a major battle in 1877 between Russia and the Ottoman Turks. He then returned to the capital, Sofia, and flew to Bucharest, Romania, next day. Turkish officials also confirmed that the accused spent an extended period in Turkey.
Peter Neumann, a researcher at London's International Centre for the Study of Radicalisation, said that regardless of the reactions among Europe's right-wing parties, the Christchurch shooter has drawn praise in online subcultures such as 8chan and 4chan. Researchers are trying to determine where those users are based and how likely it is that copycat attacks will follow.
"I think there'll be a long-term effect of the Christchurch attack," Neumann said.
"It was conceived in a way that is likely to lead to others imitating him. So, within the next decade, we might very well see at least a handful of right-wing attackers who directly reference him to justify their own actions."
