The man held over the attack on Labour MP Jo Cox has been described by neighbours as a “loner”.
Tommy Mair, 52, was detained by police near his home in Birstall, West Yorkshire.
Officers were seen searching his house not far from the scene of the brutal attack.
Neighbours said he had lived there for more than 30 years - on his own for the last two decades since the deaths of his mother and grandmother.
In 2010 a Thomas Mair, then aged 46, was quoted in his local paper talking about his mental health issues.
He volunteered at the Oakwell Hall country park in Birstall in 2010 after being a patient of the Mirfield-based Pathways Day Centre for adults with mental illness, according to a Huddersfield Examiner report at the time.
He told the paper: “I can honestly say it has done me more good than all the physiotherapy and medication in the world.
“Many people who suffer from mental illness are socially isolated and disconnected from society, feelings of worthlessness are also common, mainly caused by long-term unemployment.”
Pathways is described on the South West Yorkshire Partnership NHS Foundation Trust website as a “team that offers support to people experiencing mental health difficulties.”
Neighbours said they did not think he had a job or drove a car, and described how he would do gardening chores for local people.
David Pickles said: ”He's lived there longer than me and I've lived here since 1975. I still can't believe it. He's the last guy I would have thought of.
“He's just quiet. He kept himself to himself. He lived by himself. He's been on his own for about 20 years.
”I've never seen a lot of people visiting or anything like that, but he likes gardening. He did a lot of people's gardens round here. But he did it quietly.“
Mr Pickles said he did not know of any connection between his neighbour and Ms Cox.
Another neighbour, who did not want to be named, said: ”We've known him quite a lot of years. I've known him since I was young. He lived with his grandmother in that house, she died a few years ago. He was a bit of a loner. It's a bit of a shock - he did everyone's gardens.
“I can't say a wrong word about him, he was so quiet. It's come as a shock to everybody.”
Leah Ainley, who lives in Risedale Avenue where Mair was arrested, said armed police surrounded him and pinned him to the ground.
“I just saw them take him down and catch him,” she added.
“The police were armed. There were more than 10 of them at the end. I just looked out the window and saw them. The man was white and bald. He banged his head on the floor. He just lay on the floor while they were holding him down.
”He had a bag with him but that's all I saw, I didn't see if he was armed.“