The video game adapting the purported Christchurch footage is still being shared online. Modeled on other so-called first-person-shooter games, it tracks a gunman who enters a mosque, drawing a gun and killing anyone in his path.

In the days leading up to a court appearance by Mr. Tarrant last month, during which he pleaded not guilty to charges that included murder and terrorism, memes featuring him spiked across the message boards 4Chan and 8Chan, Mr. Decker said. Scores of boards on 8Chan are devoted to Mr. Tarrant, including forums lionizing him as St. Tarrant.

And on the day Mr. Tarrant was due in court, a user on Reddit flagged a post on 4Chan in which someone announced a plan to attack a mosque in Texas, vowing to follow the example of “our lad.” Many users flagged it to the police, and no attack occurred.

“You have these toxic communities trying to infect more mainstream congregations with xenophobia, Islamophobia and threats of mass violence,” Mr. Decker said. “The fact that it moves across platforms allows users to notify law enforcement. It definitely is a tale of two internets.”

Mr. Decker was among the consultants the New Zealand authorities met with as Ms. Ardern prepared to travel to Paris in May to issue her Christchurch Call. One question she has grappled with is how far New Zealand, an island nation of just under five million people, will go to keep the rest of the world at bay.

In the days after the Christchurch attack, local internet service providers suspended access to websites that hosted videos of the shooting and apologized for the censorship, even as they acknowledged that they could not completely prevent users from viewing the material.