Police stand guard at the crime scene in Birstall, northern England, where Labour lawmaker Jo Cox was shot on June 16. (Oli Scarff/AFP via Getty Images)

BIRSTALL, England — One day before Tommy Mair became a suspect in a killing, his neighbor in the northern English town of Birstall talked to him for the last time. “We had a normal conversation,” Diana Peters recalled Friday morning. Throughout the time she has known him, Mair seemed unhappy to her — but there were no indications he may be a supporter of neo-Nazi groups, she said.

Mair's ties to one U.S. neo-Nazi organization were revealed Thursday night, when the Southern Poverty Law Center cross-referenced its records. “We found a number of mail orders going back decades with the once-premier American neo-Nazi group National Alliance (N.A.),” the anti-hatred nonprofit organization wrote in an emailed statement.

According to the records, Mair paid the National Alliance over $620 in 1999. One of the items he purchased from the neo-Nazi organization was a manual that includes instructions for building pistols, among other publications with titles such as “Chemistry of Powder & Explosives.” Eyewitnesses described the weapon Mair is suspected of using to allegedly shoot Labour MP Jo Cox as looking homemade.

In addition to ordering National Alliance literature, Mair lived just a few miles from the home of the man who led the neo-Nazi group's British affiliate, although it was not certain they knew each other, said Nick Lowles, chief executive of the anti-extremist group Hope Not Hate.

The National Alliance was known for its virulently anti-government rhetoric, and the idea that “the state is the organ you have to destroy,” Lowles said.

Tommy Mair had long-standing ties to the National Alliance, a U.S.-based neo-Nazi organization, according to the Southern Poverty Law Center. (Jason Aldag/The Washington Post)

Although the National Alliance never established a major presence in Britain, Lowles said, it did influence the far-right's rhetoric in the United Kingdom during the 1990s by focusing the anger of sympathizers on their elected representatives.

Lowles likened the group's beliefs to those of Norwegian mass killer Anders Breivik. “In their view, the politicians are the traitors who allowed all this to happen,” he said.

So, what is behind the organization that may provide deeper insights into Mair's links to white nationalism and radicalization?

The National Alliance used to be the United States' most influential neo-Nazi organization, according to the Southern Poverty Law Center.

The organization's rise is associated with one man: William Pierce, who founded it in 1974. Only 15 years ago, the group was believed to have more than 1,000 members in the United States, spread over more than 30 states.

Apart from holding secretive meetings and working on publications, the National Alliance tried to construct its own philosophy — an ideology the group's members perceived to be a religion.

Pierce later bought a farm in Virginia that he turned into a church and the group's headquarters. Particularly in the 1990s, he started to connect with similar groups in Europe. Mair appears to have been in contact with the group at the end of that decade, according to documents provided by the Southern Poverty Law Center.

It was Pierce who held the group together for nearly 30 years, as competing organizations fell apart because of disputes between their leaders and members. But following Pierce's death in 2002, the group followed the fate of its predecessors and split into much smaller fractions with little public relevance in 2005. Its ideology, however, continues to have an impact.

One of the world's most radical neo-Nazi groups, the National Alliance stood for a hierarchical worldview that strongly opposes multiculturalism. Much of its discourse and literature focuses on the belief that the “Aryan race” is superior to other races and that whites living in Europe, North America, Australia and South Africa should create their own “living space.”

The Southern Poverty Law Center argues that the group's ideology is “genocidal” at its very core, citing propaganda material that has called for the “eradication of Jews and other races.”

British police have so far refrained from making a connection between Mair and the National Alliance's ideology. However, Cox was a strong advocate of accepting more refugee children from war-torn Syria into the United Kingdom, for instance. Moreover, the discussion surrounding next week's referendum on leaving the European Union has been dominated by the topic of immigration in recent weeks.

The populist U.K. Independence Party recently released a poster supporting Brexit (or British exit from the E.U.) showing a long line of refugees with the remark: “Breaking point” — suggesting that immigration to Britain could only be curbed if the country left the European Union. Cox was known for opposing Brexit and as an enthusiastic democrat who had only recently become a member of Parliament.

The National Alliance has repeatedly emphasized that it considers democracy a disenfranchisement of whites — and of men. Its male-centered worldview might be of particular significance because Cox was considered to be one of the rising female stars of the Labour Party.

“Feminism is a threat to our race,” the N.A. argued in its publications, according to the Anti-Defamation League (ADL), a Jewish civil-rights organization.

According to the ADL, the National Alliance's hatred toward democracy went so far that the group's members largely refused to run for office or hold ordinary campaigns.

The Southern Poverty Law Center argues that until his death in 2002, Pierce inspired several “assassins, bombers and bank robbers.” Pierce's novel “The Turner Diaries” is also considered to have been the inspiration for the 1995 bombing in Oklahoma City by Timothy McVeigh and Terry Nichols, which killed 168 people.

In total, the Southern Poverty Law Center counts 14 violent crimes that occurred from 1984 until 2005 as being connected to the National Alliance.

Investigators in Britain will now have to determine whether Thursday's attack on Cox will have to be added to that list.

Griff Witte contributed to this post.