In the document, reviewed by NBC News, the authors express admiration for Adolf Hitler and the Christchurch shooter as well as the perpetrators of high-profile mass shootings in Buffalo, New York; Isla Vista, California; Pittsburgh; Orlando; and Columbine High School in Colorado, where students were massacred in 1999.

In recent years, video of the Christchurch massacre has spread across digital platforms where extremists congregate and communicate. Officials investigating this week’s mosque attack are examining a livestream that the suspects appeared to have posted online before they died to determine whether it is authentic, three senior law enforcement officials said.

“Each attack functions as a piece of content the community consumes, references, and metabolizes into the next attack,” said Alex Goldenberg, the founder of Silent Index, a national security consultancy. “The kill count operates as a press release; the manifesto and recording, as merchandise that continues to circulate after the attackers’ deaths.

“The next attacker reads this material, attaches their own in-group signature, and produces the next entry. The lineage is not held together by an organization,” Goldenberg added. “It is held together by the recurrent production and circulation of violent content as community currency.”

Jon Lewis, a researcher at George Washington University’s program on extremism, said he was struck by the fact that the suspects were in their late teens.

“They were seemingly 10 and 12 when the Christchurch shooting happened,” Lewis said in a phone interview with NBC News. “They have simultaneously come up in an era” where “everyone in their age group is terminally online, but they’ve been consuming this content, seemingly from a very young age.”

Remer, the FBI special agent in charge, told reporters Tuesday that federal law enforcement officials were “still going through” how exactly the suspects became radicalized.

“I think that says so much about how low the barrier to entry is,” Lewis said. “I think this is, unfortunately, a really clear instance of how easy it is for this brand of do-it-yourself domestic terrorism to become reality.”