Tragic Labour MP Jo Cox 'suffered months of harassment' before being shot and stabbed
Labour MP Jo Cox, who was killed yesterday, was subjected to hate mail weeks and contacted police after receiving "malicious communications".
Police revealed that they had arrested a man in connection with the investigation in March.
Ms Cox was shot and stabbed to death in the street outside her constituency advice surgery.
A Number 10 spokeswoman said a reminder of safety guidance has been sent out to MPs after Mrs Cox was attacked in Birstall, West Yorkshire.
The advice includes steps representatives can take to stay safe when they are "out and about" and suggests if they have any concerns they should contact their local police.
Mrs Cox, the mother of two children, aged three and five, was attacked by a man reportedly shouting "Britain first" at lunchtime on Thursday.
Her death comes three months after a man was cautioned for sending her "malicious communications", but police have confirmed this was not the same man who was arrested on Thursday following the attack.
Witnesses said her assailant kicked and stabbed her and then shot her several times, the final shot aimed at her head.
The alleged gunman has been named locally as Tommy Mair, 52, who neighbours in Birstall have described as "a loner". He was arrested near the scene soon after the attack.
The MP's husband Brendan said: "Today is the beginning of a new chapter in our lives. More difficult, more painful, less joyful, less full of love.
"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.
"Jo believed in a better world and she fought for it every day of her life with an energy and a zest for life that would exhaust most people.
"She would have wanted two things above all else to happen now: one, that our precious children are bathed in love, and two, that we all unite to fight against the hatred that killed her.
"Hate doesn't have a creed, race or religion, it is poisonous.
"Jo would have no regrets about her life, she lived every day of it to the full."
Labour leader Jeremy Corbyn said the country would be "in shock at the horrific murder" of a "much-loved colleague".
The killing led to the suspension of campaigning in the EU referendum.
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Mr Corbyn and a number of other MPs attended an impromptu vigil in Parliament Square and flowers were laid nearby in tribute.
In Mrs Cox's home town, hundreds of people, including Labour colleagues Yvette Cooper and Dan Jarvis, packed into the parish church to hear the Bishop of Huddersfield, the Rt Rev Dr Jonathan Gibbs, pay tribute to someone who "gave her life for this community".
Prime Minister David Cameron said: "The death of Jo Cox is a tragedy. She was a committed and caring MP. My thoughts are with her husband Brendan and her two young children."
Downing Street said flags across Whitehall will fly at half-mast, while flags at Buckingham Palace and the Palace of Holyroodhouse in Edinburgh will also be lowered in tribute.
A Buckingham Palace spokesman said the Queen had written privately to Mr Cox.
Mrs Cox had previously complained to police about receiving malicious communications.
A Metropolitan Police spokeswoman said: "Officers received an allegation of malicious communications from Jo Cox MP, and in March 2016 arrested a man in connection with the investigation.
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"The man subsequently accepted a police caution. The man who accepted the police caution is not the man in custody in West Yorkshire."
The attack on Mrs Cox comes after female MPs said security was a "key concern" when they were surveyed last August.
A study for the Commons Administration Committee recorded that politicians had dealt with incidents involving stalkers and people with knives and guns.
In January, the Independent Parliamentary Standards Authority (Ipsa) announced that MPs were to get an ''enhanced'' security package following a wave of protests outside constituency offices and homes over the Commons vote on bombing Syria.
Clarke Rothwell, who runs a cafe near the murder scene in Birstall, described the attack, telling 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."
He said the gunman fired three shots, the final one at her head.
Britain First is the name of a far-right group which said it was "not involved and would never encourage behaviour of this sort".
Another witness, Hichem Ben Abdallah, 56, said the attacker was "kicking her as she was lying on the floor".
He said that, after a bystander intervened, the man produced a gun, stepped back and shot Mrs Cox.
Mr Abdallah, who was in the cafe next door to the library, told the Press Association the weapon looked handmade and that the man who had been wrestling with the assailant continued to do so even after he saw the gun.
Mair's house was sealed off by police who were guarding the property as forensic officers worked in the garden.
Neighbours said he had lived there for more than 30 years - on his own for the last two decades since the death of his grandmother.
One neighbour, who did not want to be named, said: "He was a bit of a loner. It's a bit of a shock - he did everyone's gardens."
Leah Ainley, who lives in Risedale Avenue, where Mair was arrested, said armed police surrounded him and pinned him to the ground.
Mrs Cox, a Remain supporter, was elected to the seat of Batley and Spen at the general election last year.
Shortly after the attack Mr Cox tweeted a picture of his wife next to the River Thames, where they lived on a houseboat.
She graduated from Cambridge University in 1995 and worked as an adviser for former Labour prime minister Gordon Brown's wife Sarah, and Baroness Kinnock.
Mrs Cox was a vocal advocate for the victims of the Syrian civil war and abstained in last autumn's contentious vote on allowing British military action in Syria.
Mr Cox, a former adviser to ex-prime minister Gordon Brown, spent Wednesday campaigning for the UK to remain in the EU.
He posted photographs of himself and the couple's children travelling along the Thames in a dinghy during a counter-protest against a pro-Brexit flotilla of vessels.
Mrs Cox set up the All Party Parliamentary Group on Syria with Conservative MP Andrew Mitchell.
Writing in the Daily Telegraph he described her as "a force of nature, a 5ft bundle of Yorkshire grit and determination absolutely committed to helping other people".
Predicting she would have one day joined the Cabinet, he said she was "uninterested" in arguments based on party lines and "just wanted to do the right thing".
The International Development Secretary also described the "fearless" way she gave the Russian ambassador a "rollicking" over his country's actions in Syria, leaving the diplomat "discomfited", and said the best word for her was "ballsy".
Birstall vicar the Rev Paul Knight described Mrs Cox as "passionate" about serving her community.
He told the BBC: "She was so proud of representing this community - it seemed to be one of her passions and her aims from early on.
"Right from the beginning, when I met her and talked with her, I could see her passion and that she really wanted to serve this community and make it a better place - improve people's lives."