The Washington Post is reporting that the attacker explicitly references US white supremacy in his manifesto:
Portions of the ghastly attack at the downtown mosque were broadcast live on social media by a man who police confirmed had also released a manifesto railing against Muslims and immigrants. The 74-page document states that he was following the example of notorious right-wing extremists, including Dylann Roof, who murdered nine black churchgoers in Charleston, S.C., in 2015.
The manifesto, littered with conspiracy theories about white birthrates and “white genocide,” is the latest sign that a lethal vision of white nationalism has spread internationally. Its title, “The Great Replacement,” echoes the rallying cry of, among others, the torch-bearing protesters who marched in Charlottesville in 2017.
The author of the manifesto also said he intended to deepen strife in the United States over gun ownership and the Second Amendment.
— Edited to remove off topic rhetoric —