The death of a British member of Parliament, who was shot outside a small-town library, has stunned Britain and halted the country’s referendum campaign on its future in the European Union.

Jo Cox, a Labour MP, was shot and stabbed around 1 p.m. on Thursday after she met with constituents in Birstall, near Leeds in northern England. Ms. Cox, 41, was taken to a hospital where police said she died of her injuries. A 52-year-old local man has been arrested.

Her death comes with a week left in the referendum campaign that has become increasingly bitter. Politicians on both sides have ratcheted up their rhetoric in recent days as polls show the Vote Leave side taking the lead after weeks of a virtual tie.

Cabinet ministers have turned on each other and this week more than 60 Conservative MPs criticized Prime Minister David Cameron and Chancellor of the Exchequer George Osborne over their handling of the campaign. Bank of England Governor Mark Carney also got into a spat this week with a senior Conservative MP who told the governor to keep quiet for the remainder of the campaign.

Ms. Cox backed the Britain is Stronger in Europe, or Remain, side. She praised the contribution of immigrants to Britain and championed the cause of war-scarred Syrian refugees.

Police have not released any motive for her killing and are investigating reports the attacker shouted a far-right slogan.

“Our working presumption … is that this is a lone incident,” said Dee Collins, acting chief constable of West Yorkshire Police. British security officials said the shooting didn’t appear to be related to international terrorism, but domestic terrorism has not been ruled out.

Both Vote Leave and Remain suspended their campaigns on Thursday.

“This is absolutely tragic and dreadful news,” Mr. Cameron said from Gibraltar where he cancelled a planned rally for Remain. “My thoughts are with Jo’s husband, Brendan, their two children and their wider family. We’ve lost a great star, she was a great campaigning MP with huge compassion and a big heart.”

Labour Leader Jeremy Corbyn said he was “utterly shocked by the news of the attack on Jo Cox. The thoughts of the whole Labour Party are with her and her family at this time.”

Boris Johnson, the former mayor of London who is leading the Vote Leave campaign, said his thoughts were with Ms. Cox’s family. “Whoever has done this, I hope they are brought to justice as soon as possible,” he added.

A small crowd gathered in the market-square area of Birstall on Thursday night, not far from where the attack took place, as police continued their investigation. Vigils were held in a local church earlier in the evening and a stack of flowers and hand-written notes lined the base of a statue.

“This is a tragic, tragic day,” said Nick Hinchcliffe, who stopped by the square to pay his respects to Ms. Cox. “She was lovely. You couldn’t wish for a nicer person. … We’re well shocked. It’s awful, really.”

Ms. Cox had been holding constituency meetings at the local library Thursday morning.

She was “was attacked by a man, who inflicted serious and sadly, ultimately fatal injuries. Subsequently, there was a further attack on a 77-year-old man, nearby who has sustained injuries that are non-life threatening,” Constable Collins said.

Aamir Tahir, who works in a nearby dry-cleaning business, said in an interview that his co-worker heard “two loud pops and then some shouting.”

Mr. Tahir, who moved to the area recently, said Ms. Cox was “an all-around nice lady” and that he was stunned by the incident. “There’s police all over the place,” he said. “This is like a little village. This is probably the first time ever in history this has ever happened here. Everyone knows each other. I’ve been here four months and I know half the town.”

Several people told local media that a man with a gun got into a scuffle with another man. When Ms. Cox tried to intervene, he began stabbing her and then pulled a gun.

“I saw people rushing down the road toward the library. I came out with a couple of people from the restaurant … we saw a man wearing a dirty white baseball cap with grey jacket start jostling with somebody,” Hichem Ben-Abdallah told Sky News. “All of a sudden this guy pulls a gun … it looked like a First World War gun or makeshift gun, not the sort of gun you see normally. He fired the first shot then I ran away and then we heard the second shot.”

Mr. Ben-Abdallah added: “He was fighting with her and wrestling with her and then the gun went off twice and then she fell between two cars and I came and saw her bleeding on the floor.”

Clarke Rothwell, another witness, told the BBC that a man carrying a gun, who “purposefully” targeted the MP, although police did not confirm this account.

“He shot this lady and then shot her again,” Mr. Rothwell said. “He leaned down. Someone was wrestling with him and he was wielding a knife and lunging at her. Three times she was shot. People were trying to help her. Then he ran off down a one-way street. Me and my mate drove round to try and find him.”

Ms. Cox was elected to Parliament last year and considered a rising star in the Labour Party. She graduated from Cambridge University and served as the head of policy for Oxfam and as an adviser to the Bill and Melinda Gates Foundation.

Britain has strict gun control and shootings are relatively rare. The overall crime rate fell by 7 per cent last year compared to the previous year and there was a 1.2-per-cent rise in gun crime. The number of homicides increased by 56 to 573 across the country in 2015.

Over the last decade the number of murders has fallen steadily while the population has continued to grow. According to the Office for National Statistics, the rate of homicides has fallen by more than one-third from 2005 to 2015.

BIRSTALL, ENGLAND — The Globe and Mail
Published Thursday, Jun. 16, 2016 9:36AM EDT
Last updated Thursday, Jun. 16, 2016 10:14PM EDT