Thomas Mair, the man arrested for the murder of British Member of Parliament Jo Cox, was reportedly a dedicated supporter of a U.S.-based neo-Nazi hate group that calls for the eradication of Jews and other races.

Mair, 52, was armed with a knife and a gun when he attacked Cox in her home village of Birstall in northern England, while shouting “Britain First” in a possible reference to the far-right Britain First party, reports said.

According to the Southern Poverty Law Center civil rights group, Mair was apparently a dedicated supporter of the National Alliance (NA), a U.S.-based neo-Nazi and white supremacist organization. Mair purportedly purchased a manual from the NA in 1999 that included instructions on how to build a pistol.

“Explicitly genocidal in its ideology, NA materials call for the eradication of the Jews and other races—what a principal foundational document describes as ‘a temporary unpleasantness’—and the creation of an all-white homeland,” the Southern Poverty Law Center said.

Cox, 41, was a popular pro-Europe Labour MP who advocated for Syrian refugees. Some commentators have speculated on whether or not the murder was linked to the contentious debate over the United Kingdom’s upcoming referendum on European Union membership.