The alleged killer of British lawmaker Jo Cox was a “dedicated supporter” of a neo-Nazi group based in the United States, a civil rights group reported Thursday (16 June).

The Southern Poverty Law Centre said that the man named by British media as the attacker, Thomas Mair, had a “long history with white nationalism”.

“According to records obtained by the Southern Poverty Law Center Mair was a dedicated supporter of the National Alliance (NA), the once premier neo-Nazi organization in the United States, for decades,” the legal advocacy group said on its website.

Cox, a 41-year-old lawmaker with the opposition Labour Party who was known for campaigning for refugee rights, was killed in a daylight attack Thursday in her home constituency in Yorkshire in northern England.

Police said an investigation was underway to establish the motive for the murder, which halted campaigning a week before Britain’s referendum on whether to leave the European Union, a debate marked by divisions over immigration.

The Southern Poverty Law Center reported that Mair had spent more than $620 on reading material from the National Alliance, a group which called for the creation of an all-white homeland and eradication of Jewish people.

Images of two invoices published on the advocacy group’s website appeared to show orders for magazines from Thomas Mair, with an address in West Yorkshire.

One handbook Mair purchased included instructions on building a gun from everyday materials, according to the Southern Poverty Law Center.

British media reported witnesses of the attack as saying that the assailant had used a gun of “old-fashioned” or “homemade” appearance.

One witness, cafe-owner Clarke Rothwell, told the Press Association that the gunman had shouted “put Britain first” repeatedly during the attack.

“Britain First” is the name of a far-right anti-immigration group, which released a statement saying it was “obviously not involved” and “would never encourage behaviour of this sort.”

Mair’s brother, Scott Mair, told the Daily Telegraph that Thomas “is not violent and is not all that political”.

“He has a history of mental illness, but he has had help,” Scott Mair said.

Separately, the newspaper reported that Mair was a subscriber to S.A. Patriot, a South African magazine published by a pro-apartheid group with an editorial stance against multiculturalism.