The alleged killer of Labour MP Jo Cox purchased a gun-making manual and Nazi literature from a far-right neo-Nazi group, new documents suggest.

52-year-old Thomas Mair was detained by police after the multiple shooting and stabbing of the MP in Birstall, West Yorkshire, on Thursday.

Ms Cox, a former aid worker, campaigned tirelessly for Syrian refugees and was a vocal supporter of the European Union and the benefits of multicultural immigration.

The Southern Poverty Law Centre, an established US civil rights group, has produced receipts and invoices bearing Mr Mair’s name that it says are from the neo-Nazi National Alliance group.

The receipts suggest Mr Mair bought $670 USD in printed material from the white supremacist group, which was until 2013 was one of the largest neo-Nazi organisations in the US.

He appears to have purchased a handbook on building improvised weapons, explosives, and incendiaries, according to the records.

In addition, the receipts suggest the purchase of Ich Kampfe, a handbook written by Adolf Hitler formerly given to all Nazi Party members.

(Southern Poverty Law Centre (Southern Poverty Law Centre)

The text of that book was drawn up by leaders of paramilitary organisations within the Nazi machine - including the infamous SS and SA - and describes their actions and methods.

The receipt documents, which date from between 1999 and 2003 show Mr Mair’s name and an address in Batley, in Ms Cox’s constituency.

The cover of Ich Kampfe, the Nazi paramilitary handbook listed in the invoices

The Independent reported yesterday that Mr Mair may have also had links to the “Springbok Club”, an organisation which has defended the white supremacist apartheid regime in South Africa.

That group, which says it is “pro-free market capitalism and patriotism and anti-political correctness”, has also campaigned against the European Union. The group has condemned Ms Cox’s killing.

Jo Cox in the Commons, June 2015

Speculation has raged about the motive for the attack after a number of separate eyewitnesses said Ms Cox’s attacker shouted “Britain first” – a longstanding far-right slogan – during the assault.

“Britain First” is also the name of a far-right organisation in Britain which recently publicly advocated “direct action” against Muslim elected officials. The group says it condemns Ms Cox’s killing.

(Southern Poverty Law Centre (Southern Poverty Law Centre)

The attacker was also said by eyewitnesses to have used a gun of antique appearance.

Eyewitnesses say Ms Cox was shot either two or three times outside her constituency advice surgery in Birstall Library. She was left lying in a pool of blood and then taken by air ambulance to Leeds General Infirmary, where she died.

Jo Cox tributes - in pictures Show all 20 1 /20 Jo Cox tributes - in pictures Jo Cox tributes - in pictures Floral tributes and candles are placed by a picture of slain Labour MP Jo Cox at a vigil in Parliament square in London AFP Jo Cox tributes - in pictures Tributes to Labour Party MP Jo Cox are placed on her houseboat in Wapping in London REUTERS Jo Cox tributes - in pictures The Union flag at half-mast on top of Portcullis House in London after Labour MP Jo Cox was shot and stabbed to death PA Jo Cox tributes - in pictures Labour leader Jeremy Corbyn (2R) and deputy leader Tom Watson (L) light candles as they attend a vigil to slain Labour MP Jo Cox in Parliament square in London AFP/Getty Images Jo Cox tributes - in pictures Labour Party leader Jeremy Corbyn and deputy leader Tom Watson (rear) arrive to leave tributes at Parliament Square PA Jo Cox tributes - in pictures People leave St Peter's Church after a vigil in memory of Jo Cox REUTERS Jo Cox tributes - in pictures Flowers left at Parliament Square opposite the Palace of Westminste, following the death of Labour MP Jo Cox PA Jo Cox tributes - in pictures People react as they look at tributes left for Labour Member of Parliament Jo Cox in Parliament Square, London REUTERS Jo Cox tributes - in pictures A man writes a message at Parliament Square PA Jo Cox tributes - in pictures People stop to look at tributes left at Parliament Square opposite the Palace of Westminster PA Jo Cox tributes - in pictures A woman arrives to lay flowers at a statue to Joseph Priestly in Birstall near to the scene where Labour MP Jo Cox was shot AFP/Getty Images Jo Cox tributes - in pictures Tributes at Parliament Square opposite the Palace of Westminster PA Jo Cox tributes - in pictures A woman places candles in tribute to Labour Party MP Jo Cox REUTERS Jo Cox tributes - in pictures A member of the public signs a memorial for British MP Jo Cox in Parliament Square, London EPA Jo Cox tributes - in pictures People sign messages of condolence for MP Jo Cox during a vigil in Parliament Square in London Getty Images Jo Cox tributes - in pictures Flags at half mast outside Scottish Parliament in Edinburgh, after Labour MP Jo Cox was shot and stabbed to death in the street outside her constituency advice surgery in Birstall PA Jo Cox tributes - in pictures People arrive in Market Square with floral tributes after the death of Jo Co Getty Images Jo Cox tributes - in pictures Floral tributes are placed in Market Square next to the statue of Joseph Priestley following the death of Jo Cox Getty Images Jo Cox tributes - in pictures Floral tributes are brought to the scene after the death of Jo Cox Getty Images Jo Cox tributes - in pictures A police officer carries bunches of flowers at the scene of the shooting of Labour MP Jo Cox in Birstall REUTERS

Eyewitness accounts from the scene paint a confused picture of events, with some reports suggesting Ms Cox intervened in a fight between two men. Others say she was directly targeted by her attacker. There has been speculation over whether Mr Mair had a history of mental health problems.

Political friends and supporters of Ms Cox gathered in parliament square on Thursday evening for an impromptu vigil.

Stood amongst tearful colleagues, Labour leader Jeremy Corbyn said in an opening address that Ms Cox was an “exemplary MP, a real servant of democracy in every way”.

Cox murder suspect arrest

A second vigil will be held on Friday night, with others also set to take place across the country.

West Yorkshire police said on Thursday that the incident was “localised” and that they were seeking nobody else either than Mr Mair in relation to the killing.