Mr. Sellner does not like the analogy. He prefers to compare his link to the mosque shooting suspect to that between leftist activists — his own parents once fell into that category — and leftist terrorists. Someone who talks about class struggle can hardly be held responsible for leftist militants then exploding a bomb, he said.

In his leather jacket and trendy horn-rimmed glasses, Mr. Sellner embodies the image makeover of the far right. He has admitted to once being a member of a neo-Nazi group, but these days, Gandhi and Martin Luther King share book shelf in his Viennese apartment. He calls himself a patriot and a democrat.

Experts like Mr. Neumann don’t buy it.

“The presentation has changed, but the message is still racist,” Mr. Neumann said. “When they talk about European identity, they mean white Christian identity. They use friendly, progressive language, but the content is the same old.”

What’s genuinely new, he said, “is that they are increasingly globally networked.”

Mr. Sellner also personifies the reach of an increasingly global movement with his close links to activists across Europe and the United States: He is engaged to Brittany Pettibone, an American YouTuber with links to the alt-right who once claimed there was a “white genocide” in South Africa.

The couple routinely appear on videos together, recently expressing admiration for Hungary’s increasingly authoritarian prime minister, Victor Orban, and Italy’s vice premier Matteo Salvini.

In the interview, Mr. Sellner described President Trump as “a catalyst for the European right” and expressed his admiration for Mr. Kurz, the Austrian chancellor, for daring to go into coalition with the far right.