A member of Parliament was gunned down outside a library in northern England as she was wrapping up a meeting with constituents on Thursday afternoon, a rare act of gun violence in a nation that strictly regulates firearms.
The lawmaker, Jo Cox, 41, who was considered a rising star in the opposition Labour Party and a passionate advocate for victims of the civil war in Syria, was shot in Birstall, a town about six miles southwest of the city of Leeds. A 77-year-old man was slightly injured in the attack.
A 52-year-old man was arrested in Ms. Cox’s killing, and the police said they were not looking for any other suspects. No motive has been established, officials said.
The suspect was identified in the British news media as Thomas Mair.
On Thursday night, the Southern Poverty Law Center reported on its website that Mr. Mair was a “dedicated supporter” of the National Alliance, a neo-Nazi organization in the United States. The center, which tracks hate groups, said Mr. Mair bought a manual from the alliance in 1999 that included instructions on how to build a pistol. It based its reporting on invoices it said it had obtained, copies of which were published on its website.
Mr. Mair’s brother Scott told The Daily Telegraph that he was “struggling to believe what has happened.”
“My brother is not violent and is not all that political. I don’t even know who he votes for. He has a history of mental illness, but he has had help,” the newspaper quoted him as saying.
Gun ownership in Britain has been tightly controlled since a 1996 massacre at a school in Scotland, and historians said it was the first time a sitting member of Parliament had been killed since 1990, when the Irish Republican Army assassinated a Conservative lawmaker, Ian Gow.
The killing occurred one week before a referendum on whether Britain should leave the European Union, and both sides immediately halted campaigning out of respect for Ms. Cox.
“The death of Jo Cox is a tragedy,” Prime Minister David Cameron of Britain wrote on Twitter, describing Ms. Cox as “a committed and caring M.P.” and “a great star.” He said, “It’s right that we’re suspending campaigning activity in this referendum.”
Jeremy Corbyn, the leader of the Labour Party, wrote on Twitter, “The whole of the Labour family, and indeed the whole country, is in shock and grief at the horrific murder of Jo Cox.”
Ms. Cox, like most other Labour politicians, supported Britain’s continued membership in the European Union. In her maiden speech in Parliament last year, she spoke of the diversity of her district, which includes Irish Catholics and Indian Muslims. “We are far more united and have far more in common than that which divides us,” she said.
Last week, she wrote on Twitter: “Immigration is a legitimate concern, but it’s not a good reason to leave the E.U.” On Wednesday, her husband, Brendan Cox, took part in a joust between campaigners from both sides, an event nicknamed the Battle of the Thames.
Mr. Cox issued a statement on Thursday evening, saying, “Hate doesn’t have a creed, race or religion, it is poisonous.”
In Birstall, where the last census recorded nearly 17,000 residents, the police cordoned off the area around Market Street, where the suspect was arrested.
The police were summoned at 12:53 p.m., and found Ms. Cox with severe injuries. She was taken by an air ambulance to Leeds General Infirmary, where paramedics tried to save her. A doctor pronounced her dead at 1:48 p.m., Dee Collins, temporary chief constable of the West Yorkshire Police, said at a news conference.
Constable Collins said that several weapons, including a firearm, had been recovered from the scene and were being analyzed. “This is a very significant investigation, with large numbers of witnesses that have been spoken to by the police at this time,” she said, adding, “We are not in a position to discuss any motive at this time.”
Speaking at the same news conference, Mark Burns-Williamson, the police and crime commissioner for West Yorkshire, described the killing as “a localized incident, albeit one that has a much wider impact.”
The BBC quoted a witness, Hithem Ben Abdallah, 56, who was in a cafe next to the library when the attack occurred, as saying he heard screaming and went outside. “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,” Mr. Ben Abdallah said.
Ms. Cox was shot while trying to intervene, he said. His account could not be independently verified.
In a telephone interview, Sanjeev Kumar, an employee at the Kwik Save store near the library where Ms. Cox met with constituents, said he saw her lying on the ground after her attacker fled.
“She was lying on the floor and two girls were helping her,” Mr. Kumar said. “She was bleeding from the mouth and nose — the ambulance was on its way. She couldn’t talk, or move, or do anything. It looked serious.”
Mr. Kumar said the attack was the sort of event that “never, never happened here.”
According to the Southern Poverty Law Center, Mr. Mair sent about $620 to the National Alliance for items from its publishing imprint, National Vanguard Books, including works that instructed readers on the chemistry of powder and explosives. Also purchased was a copy of “Ich Kampfe,” a book published by the Nazi Party in the early 1940s.
Heidi Beirich, the director of the center’s Intelligence Project, which tracks and produces reports on extremist groups, said in an interview that it had found the invoices in a database of records it maintains.
When Mr. Mair’s name surfaced on Thursday, Ms. Beirich said, the Intelligence Project matched it with invoices in its possession. She said those records were authentic because they had been leaked by members of the National Alliance. Further, she said, she was confident the records were linked to Mr. Mair because the address on the invoices matched his home address.
The West Yorkshire police could not be reached for comment early Friday on the law center’s report, and there was no indication that Mr. Mair’s apparent ties to the National Alliance had anything to do with the shooting.
Ms. Cox was elected to Parliament in May 2015 to represent Batley and Spen, a generally safe Labour seat. The district has a large number of retirees and a substantial population of people of South Asian origin.
Ms. Cox’s colleagues have described her as an eloquent and effective lawmaker. She was to celebrate her 42nd birthday next Wednesday, and had been spoken of as a potential future minister.
According to her personal website, she was born and raised around Dewsbury, England. She graduated from Cambridge in 1995, the first in her family to complete university.
Ms. Cox was involved in pro-European causes. She was an aide to Joan Walley, a British member of Parliament who supported European integration, and to Glenys Kinnock, a former Labour member of the European Parliament.
She also worked with Oxfam, the anti-hunger charity; with Sarah Brown, wife of the former Labour prime minister Gordon Brown, to reduce child and infant mortality; and with the Bill and Melinda Gates Foundation, to combat modern slavery. She was married with two children, and lived part of the time on a boat on the Thames.
The campaigns representing the two sides in next week’s European Union referendum — Britain Stronger in Europe, which favors remaining in the European Union, and Vote Leave, which advocates departure from the 28-nation bloc — halted their campaigning. Opponent of Britain’s departure said they would suspend their campaign until the weekend.
Mr. Cameron called off plans to visit Gibraltar, the British territory, to rally residents to vote in favor of remaining in the bloc.
The killing of Ms. Cox triggered an outpouring on Twitter by politicians, past and present. Although no member of Parliament has been assassinated since 1990, there have been two attacks on lawmakers meeting constituents, as Ms. Cox was doing.
In January 2000, a mentally ill man attacked a Liberal Democratic lawmaker, Nigel Jones, with a samurai sword, killing an aide to Mr. Jones.
In September 2010, Stephen Timms, a Labour member of Parliament for the East Ham district in London, was stabbed and critically injured by an Islamist extremist angered by his support for the 2003 invasion of Iraq.
Gabrielle Giffords, a former member of Congress who survived an assassination attempt in 2011 by a gunman who killed six others, wrote on Twitter that she was “absolutely sickened to hear of the assassination of Jo Cox.” She added: “She was young, courageous, and hardworking. A rising star, mother, and wife.”