Brenton Tarrant, the white supremacist suspected of killing at least 50 people last week in two New Zealand mosques, saw his murderous rampage as part of a morbid historical fantasy.

On his drive into the city of Christchurch, he listened to a nationalist Serb song that glorified Radovan Karadzic, the Bosnian Serb political leader jailed on genocide charges and war crimes for his role in atrocities against Bosnian Muslims in the 1990s. Images of Tarrant’s weaponry showed that he had scrawled the names of a number of European Christian commanders — spanning many centuries — who warred against predominantly Muslim armies. He had apparently named one of his guns “Turkofagos,” or “Turk-eater” in Greek.

There’s nothing particularly original about Tarrant’s obsession with a long history of Europeans killing Muslims. Anders Breivik, the Norwegian terrorist who killed 77 people in a shocking string of attacks in 2011, styled himself as a crusading Templar knight and issued a manifesto that contained hundreds of references to conflicts in the Balkans — a region he and Tarrant both clearly viewed as a fault line between Islam and the West.

Investigators in a number of countries, including Bulgaria, Croatia, Serbia, Romania, and Greece, are now tracking the path of trips to the Balkans that Tarrant made between 2016 and 2018. According to the Wall Street Journal, his stops included an 11th-century monastery where a Romanian prince once prayed for victory against Ottoman troops and a mountain pass where a band of Russian and Bulgarian soldiers repelled a much larger Turkish army in 1877. These pilgrimages, combined with his horror at the sight of numerous ordinary Muslims living their lives in France, seemed to underpin his extremist worldview.