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he man held over the killing of Labour MP Jo Cox is "not a violent man and is not that political", his brother has said.
Named locally as Thomas Mair, the 52-year-old was arrested by police close to the home where he lived alone in Birstall, West Yorkshire.
Witnesses reported that during the attack he shouted "Britain first" twice.
Scott Mair, 50, told reporters on Thursday that his brother had a "history of mental illness" but that "he has had help".
Police have so far refused to discuss the possible motive behind the killing, despite reports that Mr Mair may have had sympathy for far-right groups.
Scott Mair told the Sun: "We are struggling to believe what has happened. My brother is not a violent man and is not that political. We don't even know who he votes for. I am visibly shaken at this news. I am so sorry for the MP and her family."
Mr Mair's half-brother, Duane St Louis, 41, told ITV he believed his brother "wouldn't hurt a fly".
"He's never expressed any views about Britain, or politics or racist tendencies. I'm mixed race and I'm his half-brother; we got on well," he said.
Friends and neighbours of Mr Mair's mother Mary Goodall told the BBC she was devastated by the news and had been unable to sleep.
They said they did not know him as someone who had long-term mental health issues, although he may have been treated for depression a long time ago.
Known to them as Tom, the neighbours said he went to visit Mrs Goodall once or twice a week to do shopping and cleaning.
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Image captionPolice said a man had been arrested in Market Street near the scene of the attack in Birstall
Police forensics teams searched Mr Mair's home on Thursday while his neighbours in Batley also described their shock over the news.
One man said: "He's a very quiet person. He likes his gardening and stuff like that. He was a man of few words really."
Another neighbour said she had seen him outside the house on the morning of the attack. "He just walked past like he does," she said.
Friend and neighbour David Hallas said: "When his image came up on the screen me and my wife said, 'not in a million years'. Of all the people in Birstall that I know, he would've been at the bottom of the list."
He described him as shy, a loner, a "gentle, kind chap", a well-read man with whom he had talked politics and someone who never hinted at extremist views.
West Yorkshire Police said they would not speculate about the motive but the area's Police and Crime Commissioner, Mark Burns Williamson called the killing a "localised incident".
There have been a number of reports saying 52-year-old Mr Mair may as a young man have had right-wing sympathies. His name appears on a website listing him as a past subscriber to the SA Patriot magazine - a South African neo-Nazi publication.
The magazine issued a statement confirming that he appeared to have been a subscriber back in the 1980s, but said his subscription was not renewed and that those involved in the publication had never met him.
Meanwhile a US civil rights group, the Southern Poverty Law Center (SPLC), claimed it had obtained records showing Mr Mair had links with the US neo-Nazi organisation National Alliance (NA) in 1
http://www.bbc.co.uk/news/uk-36558386