The 41-year-old is the first British lawmaker to be killed in office since Conservative MP Ian Gow was assassinated by the IRA in 1990.
Cox was shot and stabbed in the street in Birstall, near Leeds in northern England, the Britain Press Association reported, citing eyewitnesses. She had just finished a regular public meeting with constituents.
A 52-year-old man was arrested shortly afterward close to the scene of the attack, said Dee Collins, Temporary Chief Constable of West Yorkshire Police. Weapons, including a firearm, were recovered, she said.
At 1.48 p.m. local time, less than an hour after the attack, Cox was pronounced dead by a doctor working with a paramedic crew.
The MP’s bereaved husband, Brendan Cox, said that 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.”
Witness: Attacker yelled ‘put Britain first’
Police have not commented on the circumstances surrounding the attack and a motive wasn’t immediately clear.
But a witness claimed that the assailant who attacked Cox was yelling “put Britain first,” the Press Association reported.
Clarke Rothwell, who runs a cafe near the murder scene, told the Press Association: “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.”
The gunman fired three shots, the final one at her head, he told the Press Association.
Another witness, Hichem Ben Abdallah, said the attacker kicked Cox as she lay on the ground until a bystander intervened and the attacker produced a gun and shot her, the Press Association reported.
“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,” he said.
The gunman was wrestling with Cox “and then the gun went off twice,” Abdallah told the Press Association. “I came and saw her bleeding on the floor.”
Footage circulated of the suspect on the ground after being apprehended by uniformed police officers.
Tensions high ahead of referendum
Britain is one week away from a momentous public referendum on whether to stay or leave the European Union.
Passions have been running high and the political environment has become toxic, with MPs accused of lying and making up their arguments on both sides of the debate.
Cox, who was elected a member of Parliament for Batley and Spen in Yorkshire last year, was a supporter of Britain voting to remain in the EU.
Her husband and children were on a “Stronger In” boat campaigning on the River Thames Wednesday, she tweeted at the time.
Campaign groups on both sides of the debate on next week’s referendum announced they were halting their operations Thursday in the wake of the attack.
Britain First, a fringe nationalist political party, issued a statement denying any connection in the attack in light of reports about the assailant’s comments. The statement said the party “would never encourage behavior of this sort.”
News of the killing has sent shock waves through Britain, where attacks on politicians are extremely rare and the slaying of a lawmaker is without parallel in recent history.
Vigils were held
British Prime Minister David Cameron, who canceled a scheduled “Stronger In” pro-EU rally in Gibraltar Thursday following the attack, described the killing as “dreadful, dreadful news.”
“She had a huge heart. She was a very compassionate, caring MP. She was a bright star — no doubt about it. A star for her constituents, a star in parliament, and a star right across the House.”
Labour leader Jeremy Corbyn said the whole country would be in shock at the “horrific murder” of a politician he said was “universally liked at Westminster.”
“Jo Cox died doing her public duty at the heart of our democracy, listening to and representing the people she was elected to serve. It is a profoundly important cause for us all,” he said in a statement.
London Mayor Sadiq Khan paid tribute to Cox, writing on Facebook that in her year as an MP, “she made more impact than others make in a whole parliamentary career.”
“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.
“I knew that she would bring all her passion for social justice and equality to Parliament and fight just as hard for her own community in Westminster as she has for so many others around the world.”
Fellow Labour MP Mike Gapes described her as a rising star in the party, “one of the most effective of the newly elected Labour MPs last year.”
“She’s had a big impact already,” Gapes told CNN. “She’s been one of the most outspoken people calling for more to be done to stop (Syrian President Bashar al-Assad’s) barrel bombing in Syria and to get humanitarian corridors to help for the refugees from Syria.”
Before entering Parliament last year, Cox worked for aid agency Oxfam and for a pro-European campaign organization, according to her website.
Police say said no one else is being sought in relation to the attack. A 77-year-old man was also assaulted following the attack on Cox, but his injuries were not life-threatening, Collins said.
Attack has similarities to shooting of U.S. congresswoman
The attack bore some similarities to the 2011 shooting of a young U.S. congresswoman.
U.S. Rep. Gabby Giffords, then 40, was shot in the head at a “Congress on Your Corner” constituent event outside an Arizona grocery store.
Giffords, the main target of gunman Jared Lee Loughner, survived, but six people were killed. Giffords was hospitalized in critical condition and spent months recovering.
She returned to the floor of the House of Representatives seven months after the attack. But in 2012, she announced her resignation to focus on her health and recovery. She continues to struggle with the effects of the shooting today.