Police have arrested a man, Thomas Mair, for stabbing and shooting British MP Jo Cox multiple times, thereby leading her to death.

The British lawmaker was in a meeting with constituents on Thursday when she was attacked by Mair. The incident happened just day before Britain is scheduled to cast votes for a contentious referendum whether it would continue as a member of the European Union anymore. According to The Independent, the killer is believed to have strong links with a hard-right group in London. This group is focused on making Britain leave the European Union and has been campaigning for the same for many years.

Thomas Mair has been a supporter of an online publication of the Springbok Club. It is an organization that works and favors the white supremacist apartheid regime in South Africa. Cox vocally supported Britain to retain its position in the EU, which is believed to be the main cause of his murder. Witnesses have revealed that the killer shouted “British First” while he shot and stabbed the MP. British First is an organization that was formed by the former British National Party members but it dejected indication of its connections with Mair.

The connection between the Springbok Club and Mair can be traced back to ten years now when its online publication The Springbok Cyber Newsletter inquired about its earliest “S.A. Patriot” supporter’s whereabouts. This was when the killer’s address changed. A recent article published by the newsletter in June 2016 was fully dedicated to the British and the EU referendum.

“On Thursday, 23rd June 2016 all British voters will have the opportunity to vote on the future of their country. They can vote either to remain entrapped in the artificial and retrograde European Union or to regain their sovereign independence…,” the article read.  “But we should not only be concentrating at the negative aspects of remaining in the EU. Our campaign should be primarily positive and optimistic for a future outside the EU,” it added.

Time.com adds Jo Cox was married to another humanitarian campaigner Brendan Cox, who tweeted a simple text accompanied by his wife’s photo following the incident.