(NewsNation) — Former CIA officer and FBI special agent Tracey Walder joined NewsNation Prime on Sunday to offer her analysis of the mass shooting in Buffalo, New York. The suspect in that attack, who has been identified as 18-year-old Payton Gendron of Conklin, New York, is currently in custody.

“The core of his idea was an idea called ‘the Great Replacement theory,'” she said of the alleged attacker, pointing to a manifesto posted online shortly before the attack.

That theory is a mainstay among white nationalists and members of other far-right organizations. It maintains that the white population of the United States will be replaced with non-white people through immigration and uneven birth rates.

“I think where it really came to its forefront was back in Charlottesville, when we saw them sort of chanting in the streets things that went along with the Great Replacement theory,” she said.

Walder also noted that the gun that the suspect is alleged to have used appeared to have been purchased legally.

“However, it sounds like he clearly made adjustments to the gun that are illegal,” she added. Buffalo Mayor Byron Brown has called for “sensible gun control” following the shooting.