As tributes pour in for British lawmaker Jo Cox, new information is emerging about potential extremist group ties held for years by the suspected killer of the "champion of refugees" and humanitarian who "fought for a better world." 

Cox, a Labour Party MP and 41-year-old mother of two, died Thursday after being shot and stabbed while meeting with her constituents in Birstall, West Yorkshire.

According to Labour Party Leader Jeremy Corbyn, Cox was killed by a "well of hatred." Corbyn joined Prime Minister David Cameron on Friday to pay tribute to Cox in Birstall.

The attack occurred one week ahead of the so-called Brexit referendum, when Britons will say whether or not they wish to remain in the European Union. The death of pro-EU Cox prompted both the official Leave and Remain campaigns to be halted for a second day on Friday.

Witnesses reportedly heard the attacker shout "Britain first," perhaps a reference to the far-right, anti-immigrant, anti-Muslim party.

The suspect has been identified in multiple media reports as 52-year-old Thomas Mair, a constituent of Cox. He is now in police custody. They have not yet cited any motive.

Neighbors describe Mair as a loner, while Mair himself also spoke to a local paper six years ago of suffering from mental health issues.

According to the U.S.-based Southern Poverty Law Center, (SPLC):

Mair was a dedicated supporter of the National Alliance (NA), the once premier neo-Nazi organization in the United States, for decades. Mair purchased a manual from the NA in 1999 that included instructions on how to build a pistol.

Mair [...] sent just over $620 to the NA, according to invoices for goods purchased from National Vanguard Books, the NA’s printing imprint. Mair purchased subscriptions for periodicals published by the imprint and he bought works that instruct readers on the “Chemistry of Powder & Explosives,” “Incendiaries,” and a work called “Improvised Munitions Handbook." Under “Section III, No. 9” (page 125) of that handbook, there are detailed instructions for constructing a “Pipe Pistol For .38 Caliber Ammunition” from components that can be purchased from nearly any hardware store.

In addition, according to reporting by the UK's Daily Telegraph, Mair was also a subscriber to the South African magazine S. A. Patriot, which was published by the pro-apartheid White Rhino Club.

Such ties could suggest a connection to the fatal attack on Cox.  As Foreign Policy In Focus columnist Conn Hallinan wrote, "The awkwardly named 'Brexit' has stirred up a witches' brew of xenophobia, racism, and nationalism." The Washington Post reports on this backdrop as well:

Nick Lowles, chief executive of the British-based anti-extremist group Hope Not Hate, said Mair had affiliation with far-right groups that stretched back decades, although he was not a prominent player in any of them.

It remains unclear whether the attack had any links to debates over immigration ahead of the E.U. vote next Thursday, Lowles said. But he described the current atmosphere as “increasingly toxic.”

“That leads to increased prejudice. That leads to increased hate. And, at some stage, that leads to violence,” he said. “Whatever the outcome next week, the U.K. has become a much more intolerant and divided society. It’s going to take a long time to heal.”

USA Today notes that the last time a serving member of the UK Parliament was killed was in 1990, "when Conservative lawmaker Ian Gow died after the Irish Republican Army placed a booby-trap bomb under his car outside his English home"