LONDON — (CNN) Investigators are searching for answers a day after a man killed British politician Jo Cox, in a brazen attack on the street in the parliament member’s own constituency in northern England.

Cox, a member of parliament, was stabbed and shot in Birstall on Thursday after a meeting with constituents, the Press Association reported, citing witnesses.

The 41-year-old mother of two died in hospital hours after the attack, which comes a week before a crucial referendum on whether Britain should stay in or leave the European Union.

While it is unclear if it the killing had any connection with the upcoming vote, Cox was openly in favor of remaining part of the bloc, and was an avid campaigner for the rights of refugees.

The public killing has shocked the nation, which has tight gun laws, and has put the brakes on the debate around the referendum, just as both sides began ratcheting up their campaigns. On Thursday afternoon, both camps suspended campaigning out of respect for Cox. The BBC canceled a TV debate on the referendum scheduled for Friday night.

Man ‘supporting white supremacy’ in custody

Police have arrested a man in connection with the killing, West Yorkshire’s Police and Crime Commissioner Mark Burns Williamson said during a news conference Thursday. The Press Association reported that Tommy Mair, 52, had been detained.

“I must stress that investigations are ongoing, a man has been arrested, and we need to let the police do their job in understanding exactly what has happened that led up to this hugely tragic incident and channel all our thoughts into supporting the families and communities affected,” he said.

CNN witnessed police searching a home in Birstall Thursday evening, and a neighbor, who asked not to be identified, confirmed that the house belonged to Mair.

The U.S.-based Southern Poverty Law Center has published what it says are documents showing that Mair has a history of purchasing material from the National Alliance white supremacist organization, based in the United States.

The SPLC published copies of receipts and a 2013 subscription to the NA’s publication National Vanguard, as well as receipts from 1999 showing purchases for a neo-nazi book “Ich Kampfe” and the “Improvised Munitions Handbook,” along with other books.

Neighbors describe Mair as a loner and a “quiet” man who tended their gardens.

Mair had lived in the house for a long time, a neighbor said. She didn’t believe he had a regular job, and saw him around at odd hours.

Weapons, including a firearm, were recovered at the scene, said West Yorkshire Police Chief Constable Dee Collins.

Mair’s half brother Duane St Louis told ITV news he’d never seen any sign that Mair had an interest in knives or guns, and said he had “never been in trouble” and “wouldn’t hurt a fly.”

Authorities have not commented on the circumstances surrounding the attack and a motive wasn’t immediately clear.

‘Put Britain first’

In a nation with such controlled gun laws and low homicides linked to weapons, Cox’s attack was especially jarring.

Witness Clarke Rothwell, who runs a cafe near the crime scene, said the gunman yelled something before he shot Cox three times, the Press Association reported.

“He was shouting ‘Put Britain first.’ He shouted it about two or three times. He said it before he shot her and after he shot her,” Rothwell said.

Britain First, a fringe nationalist political party that wants the nation to leave the European Union, denied any connection to the attack following reports of the assailant’s comments.

In a statement, it said the party “would never encourage behavior of this sort.”

Security concerns

Attacks against British politicians are so rare, lawmakers at Cox’s level are mostly unaccompanied by a security detail.

The killing has raised questions over whether more can be done to protect British MPs, more than half of whom say they have been stalked or harassed, according to the Journal of Forensic Psychiatry & Psychology.

Of 239 MPs surveyed, 53% said they had been stalked or harassed, 81% said they experienced “intrusive or aggressive behaviors,” and 18% said they had been attacked or subjected to attempted attack, the journal reported in January.

Younger and newer MPs were more likely to be stalked, but more serious incidents were associated with more senior lawmakers.

Passionate campaign

During the heated campaign leading up to a June 23 referendum on the European Union, parliament members have been subjected to public ire, many accused of lying and making up arguments.

For now, that has all come to a halt.

“It’s right that all campaigning has been stopped after the terrible attack on Jo Cox,” Prime Minister David Cameron tweeted Thursday.

Cox championed for the rights of immigrants, saying leaving the EU would not reduce the number of refugees coming to the UK. She was particular vocal on Syrian child refugees, playing a part in the government’s decision to allow a group of child refugees in the country.

Supporters calling for an exit from the EU have cited the high number of immigrants as one of the reasons, with tensions high on both sides of the debate.

In one of Cox’s last posts on social media, she said while immigration “is a legitimate concern,” it’s not a good reason for the UK to leave the EU.

Fellow Labour MP Mike Gapes described her as a rising star in the party.

“She’s had a big impact already,” he said. “She’s been one of the most outspoken people calling for more to be done to stop barrel bombing in Syria and to get humanitarian corridors to help for the refugees from Syria.”

Witness: Attacker kicked Cox

Witness Hichem Ben Abdallah said the attacker kicked Cox as she lay on the ground. A bystander intervened and the attacker produced a gun and shot her, Abdallah told the Press Association.

“There was a guy who was being very brave and another guy with a white baseball cap who he was trying to control and the man in the baseball cap suddenly pulled a gun from his bag,” Abdallah said.

“The gun went off twice,” Abdallah said. “I came and saw her bleeding on the floor.”

Police said no one else is being sought in relation to the attack.

Husband: Death starts ‘painful’ chapter

The MP’s husband, Brendan Cox, said her killing marked the “beginning of a new chapter in our lives.”

“More difficult, more painful, less joyful, less full of love,” he said.

“I and Jo’s friends and family are going to work every moment of our lives to love and nurture our kids and to fight against the hate that killed Jo.”

Crowds gathered for vigils in Birstall and in London’s Parliament Square to pay tribute to the slain politician.

Tributes pour in

London Mayor Sadiq Khan said Cox “made more impact than others make in a whole parliamentary career” during her year in parliament.

“Everyone who met Jo knew she was special. I knew her from her time as a fearless campaigner working on behalf of some of the world’s poorest and most marginalized people,” he wrote.

Democratic presidential candidate Hillary Clinton paid tribute to Cox.

“This is how we must honor Jo Cox — by rejecting bigotry in all its forms, and instead embracing, as she always did, everything that binds us together,” she said in a statement.

The attack bore some similarities to the 2011 shooting of a U.S. congresswoman Gabby Giffords.

Giffords, then 40, was shot in the head at a constituent event outside an Arizona grocery store.

While she survived and spent months recovering in hospital, six people were killed. She resigned a year later to focus on her health and continues to struggle with the effects of the shooting.

The attack resonated with Giffords, who said she was “absolutely sickened” by it.

“Just like January 8, 2011, did not deter America from its founding ideals, the British principle of pluralism and the nation’s democratic institutions will endure,” she said in a statement.

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