LONDON (AP) — A British lawmaker who campaigned for the country to stay in the European Union was killed Thursday by a gun- and knife-wielding attacker in her small-town constituency, a tragedy that brought the country’s fierce, divisive referendum campaign to a shocked standstill.

Jo Cox, a 41-year-old Labour Party legislator who praised the contribution of immigrants to Britain and championed the cause of war-scarred Syrian refugees, was attacked outside a library in Birstall, northern England, after a regular meeting with constituents.

Police would not speculate on the attacker’s motive.

Witnesses described a man shooting Cox several times and appearing also to stab her as she lay on the pavement. Police said they had arrested a 52-year-old man and were not looking for anyone else.

“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.

Residents identified the suspect to the BBC and other media as Birstall resident Tommy Mair. Neighbors said Mair was a quiet man who did gardening jobs for local people.

Violence against British politicians has been rare since Northern Ireland’s late-1990s peace agreement, and figures from all parts of the political spectrum expressed deep shock.

Cox is the first serving member of Parliament to be killed in a quarter century, and British politicians of all stripes expressed deep shock.

Both the Vote Leave and Britain Stronger in Europe campaigns suspended activity ahead of next week’s vote over whether Britain should remain a part of the 28- member bloc.

Prime Minister David Cameron canceled a speech and rally in Gibraltar, and flags on British government buildings were lowered to half mast.

“This is absolutely tragic and dreadful news,” Cameron said. “We have lost a great star. She was a great campaigning MP with huge compassion and a big heart.”

Labour leader Jeremy Corbyn said “the whole of the Labour Party and Labour family – and indeed the whole country – will be in shock at the horrific murder of Jo Cox today.”

“Jo died doing her public duty at the heart of our democracy, listening to and representing the people she was elected to serve,” he said.

It was unclear whether the attack was directly related to the referendum campaign, which has stirred deep passions as Britons argue over their place in Europe, the scale of immigration and the future of their country.

The rival sides in the referendum have been canvassing feverishly ahead of what is expected to be a close vote, and Cox had been campaigning for the “remain” camp.

It wasn’t clear whether Cox had been deliberately targeted.

Britain’s Press Association news agency quoted eyewitness Hithem Ben Abdallah as saying Cox got involved in a scuffle between two men in Birstall, a small market town some 200 miles (320 kilometers) north of London.

Abdallah said one of the men was fighting with Cox and then a gun went off twice and “she fell between two cars and I came and saw her bleeding” on the ground.

Clarke Rothwell, who runs a nearby cafe, told the BBC he believed Cox had been shot and stabbed multiple times.