Just minutes before the suspect in the Walmart mass killing began his horrific rampage, a document praising the Christchurch mosque shootings appeared online.

Patrick Crusius, 21, of Allen, Texas, has been identified as the suspect in the Saturday Walmart shooting, which killed 20 people and injured a further 26.

The document begins: “In general, I support the Christchurch shooter and his manifesto. This attack is a response to the Hispanic invasion of Texas.”

What follows is more than 2100 paranoid and illogical ramblings, pontificating on a “Hispanic invasion” and the “mass immigration” that is “detrimental to the future of America”.

Alleged gunman Patrick Crusius. Photo: Facebook

The document called for an American apartheid, where each race has one state to live in and warned that white Americans would lose their jobs and democracy because of foreigners.

It praised President Donald Trump and argued that European “patriots” didn’t have the gun rights they needed to “repel” immigrants.

The document was posted on the website 8chan, while the FBI has confirmed it is investigating the link between the rant and Mr Crusius.

The shooting happened less than 24 hours before another gunman opened fire in Dayton Ohio, killing nine people and injuring dozens more.

US president Donald Trump has ordered flags to be lowered to half mast following the two shocking attacks as authorities said they were treating the Texas massacre as domestic terrorism and would seek the death penalty.

Today, I authorized the lowering of the flags to half-staff at all Federal Government buildings in honor of the victims of the tragedies in El Paso, Texas, and Dayton, Ohio….

— Donald J. Trump (@realDonaldTrump) August 4, 2019

From Christchurch to El Paso and across the Atlantic, far-right extremists with anti-immigration beliefs have become louder and more emboldened by their ideology in the past few years.

In April, a gunman opened fire at a synagogue in California, killing one person. Beforehand, he also uploaded a Christchurch-inspired rant to 8chan.

Christchurch’s Muslim community prays after the March shooting. Photo: Getty

It also drew inspiration from a massacre at a synagogue in Pittsburgh last year, where gunman Robert Bowers allegedly took the lives of 11 people.

Before that attack, Mr Bowers posted on social media about immigrants “invading” the US, and labelled Jewish people the “enemy of white people”.

He has pleaded not guilty to 36 charges, including 11 counts of homicide.

In 2017, a right-wing nationalist opened fire at a mosque in Quebec City, Canada, killing six people.

Alexandre Bissonnette, who pleaded guilty, told police he decided to attack the group of Muslims after Prime Minister Justin Trudeau said his government would welcome refugees.

And in 2018, nationalist Gregory Bush killed two black shoppers in a grocery store.

As Mr Bush was about to drive off, Louisville resident Ed Harrell said he went to grab his revolver before Mr Bush told him: “Don’t shoot me. I won’t shoot you. Whites don’t shoot whites.”

A conspiracy takes root

While the targets of these attacks differ – some are Muslims, some are black, some are Jewish – they’ve all been inspired by the same extreme conspiracy theory.

These extremists are indoctrinated by the notion that a “great replacement” is under way, as Caucasian birth rates slip behind those of other ethnic groups.

They call it a white genocide – driven by immigration, falling fertility rates, feminism and gay rights.

Eleven people were killed in the 2018 Pittsburgh synagogue shooting. Photo: AAP

Months after the Christchurch shootings the UK-based Institute for Strategic Dialogue (ISD) found that language related to the ideology had filtered through to mainstream conversations.

Between 2012 and 2019, more than 1.48 million posts on social media mentioned the ‘great replacement’ in English, French or German.

“It can be observed that debate reached its peak after the Christchurch attack, which propelled the term ‘Great Replacement’ to mainstream attention,” the report reads.

“The centrality of the so-called Great Replacement theory to the Christchurch attack requires policymakers to reassess the threat posed by groups who continue to espouse and spread this theory online.”

The report highlighted politicians using the term ‘invaders’ to describe Muslims and immigrants, from Hungarian Prime Minister Viktor Orbán to Italian Deputy Prime Minister Matteo Salvini, as well as Mr Trump.

Each of them mirrored the language used by proponents of the conspiracy theory, the report noted.

Anti-hate protesters hit the streets in Pittsburgh after the shooting. Photo: AAP

The theory is chiefly the work of Frenchmen Renaud Camus, who in 2012 wrote a book that described how white Christian Europe was in his view being destroyed by black and brown immigrants.

In an article for Time, ISD researchers Jacob Davey and Julia Ebner wrote that centrist politicians and commentators had failed to control the immigration conversation.

“A lack of willingness to openly discuss and acknowledge common concerns around the societal changes wrought by migration has left a vacuum, which extremists fill to their advantage,” they wrote.

“They have tapped into anxieties that remained unaddressed by the moderate middle, and therefore monopolise the debate as a result.”

Calling for moderates to “stop burying their heads in the sand”, the authors argue for migration courses in schools and politicians to take back the narrative on immigration.

“The political centre needs to demonstrate leadership in regards to the history, dynamics and consequences of migration, and actively push back against voices which promote the theory, either through dog-whistle tactics or explicit endorsement,” they wrote.