Jo Cox
Jo Cox Reuters

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.

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 behavior of this sort."

Meanwhile, Democratic presidential candidate Hillary Clinton condemned the killing of Cox on Thursday night.

“I am horrified by the assassination of British MP Jo Cox, murdered earlier today in her district in Northern England. By all accounts, she was a rising star. Her maiden speech in Parliament celebrated the diversity of her beloved Yorkshire constituency, and passionately made the case that there is more that unites us than divides us. It is cruel and terrible that her life was cut short by a violent act of political intolerance,” she said in a statement.

“It is critical that the United States and Britain, two of the world's oldest and greatest democracies, stand together against hatred and violence,” Clinton added.

Knesset Speaker MK Yuli Edelstein sent a letter of condolence to the British public and his UK counterpart, following the assassination of Cox, saying he was "deeply shocked" over the young MP's tragic death, and sending his "condolences to my counterpart, Speaker of the House of Commons John Bercow, and to the British people."

"A murder committed against an elected representative, for expressing an opinion, is a vile act, and a warning sign for any society, including ours in Israel," Edelstein continued, referring to reports the former human rights campaigner had been targeted by a right-wing extremist for her political views.

"Freedom of Expression is the bedrock of our societies, and such violence is an abhorrent phenomenon that must be stamped out."

AFP contributed to this report.