LONDON — Investigators were searching for a motive Friday behind the killing of a British lawmaker whose death stunned the nation and sparked questions over whether a divisive EU membership debate may have played a role.
A U.S. civil-rights group said the man suspected of the attack had ties to an American white supremacist network — adding to speculation that the fatal attack on Jo Cox could have been inspired by her pro-immigration views.
SEE ALSO: British lawmaker shot dead, EU referendum campaigns suspended
Cox — a rising star in the Labour Party known for her social-justice advocacy and calls for Britain to remain in the EU — was shot and stabbed in broad daylight Thursday in her northern England constituency.
Campaigning around the fiercely-debated upcoming referendum on Britain's European Union membership was suspended in wake of Cox's death.
See more from this this tragic case:
Police have not suggested a motive for the killing or named the 52-year-old man they have in custody — but local media including NBC's U.K. partner ITV News identified the suspect as Thomas Mair, whose half-brother expressed shock at his arrest. He told ITV that Mair's mother "can't understand" what may have happened. The BBC and Associated Press also have named Mair, citing local sources.
The U.S.-based Southern Poverty Law Center said Mair had popped up in its databases and was for decades "a longtime" and "dedicated" supporter of the neo-Nazi National Alliance.
It added that Mair had purchased titles including the "Improvised Munitions Handbook" — which gave instructions on how to build a pistol — from the Nazi Alliance's publishing imprint.
The National Alliance's late founder was known for racist novels — including a fictional story of a white man fighting in a race war, according to the Southern Poverty Law Center.
A man by the name of Thomas Mair from West Yorkshire also named as a former subscriber and supporter of the pro-Apartheid publication SA Patriot, according to the Associated Press.
Thursday's attack in West Yorkshire's small community of Birstall came hours after a high-profile lawmaker in the "Leave" campaign unveiled a controversial poster blaming the refugee crisis on the EU and featuring a long line of migrants.
"We... have far more in common than that which divides us"
One year ago, Jo Cox made her first parliamentary speechhttps://t.co/M9Qd9MrAf2
— PA Media (@PA) June 16, 2016
Cox had frequently pleaded for the campaign not to be hijacked by immigration fears, writing in a commentary for her local Yorkshire Post newspaper last week that "immigration is a legitimate concern, but it's not a good reason to leave the EU."
While campaigning on the "Brexit" referendum has been suspended, the poll is still due to be held next Thursday.
Britain's pound sterling currency recovered in value against the dollar after the tragedy, which analysts said could hurt the momentum of the "Leave" campaign.
Immigration fears had boosted the "Leave" campaign in recent days, giving it a poll lead for the first time after months in which the "Remain" camp was comfortably ahead.
Alan Ruskin, global co-head of foreign exchange research at Deutsche Bank, said it was clear that people were discussing whether there was a chance Cox's death would influence a Brexit vote in favor of "Remain."
"It is a tragic event all around," he said. "There is a sense, there is an immediate emotional reaction, but there is still a week before the referendum itself."
Cox's death has deeply shaken Britain, where gun controls are strict and shootings are rare.
U.K. Prime Minister David Cameron led tributes to Cox in Birstall on Friday after laying flowers at the scene of the attack.
See how the British people responded to the shocking attack:
In a rare display of political unity, Cameron stood side-by-side with opposition leader Jeremy Corbyn to mourn Cox's death.
"Today our nation is rightly shocked. And I think it is a moment to stand back and think," Cameron said. "Where we see hatred, where we find division, where we see intolerance, we must drive it out of our politics and out of our public life and out of our communities.
Corbyn, meanwhile, called the slaying of Cox was "an attack on democracy."
"It's the well of hatred that killed her," he added, saying that Cameron had accepted his request to recall Parliament from a break on Monday so lawmakers can pay tribute to Cox.
The attack has also resonated outside of the U.K., prompting outpourings of condolences and solidarity from officials around the world.
Presumptive Democratic presidential nominee Hillary Clinton said in a statement that she was "horrified by the assassination" and that Cox was "a rising star."
"It is cruel and terrible that her life was cut short by a violent act of political intolerance," she said.