Britain's MI5 is investigating potential links between the alleged Christchurch shooter and domestic extremist groups after the accused gunman made an explicit threat against London mayor Sadiq Khan.

The Sunday Times has reported this morning that MI5 - tasked with the United Kingdom's counter-intelligence and security - has taken the lead in the investigation.

The particular UK focus was linked by the Sunday Times to the alleged gunman's so-called manifesto, distributed by email and published online shortly before Friday's attack began.

The manifesto said specifically named Khan and terror attacks in Britain. The Herald has decided not to publish any of the comments in the manifesto.

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Source: Daily Telegraph UK
Source: Daily Telegraph UK

Brenton Tarrant, 28, has been charged with murder after the Deans Ave mosque and Linwood Ave mosque shootings that killed 50 people. He appeared in the Christchurch District Court yesterday, where he made no plea. He is due to appear again next month.

Tarrant is believed to have travelled to Britain on a two-month tour of Europe which galvinised his extremism, the UK's Daily Telegraph reported.

The suspected gunman toured the continent and came to Britain to stay for up to a fortnight in 2017.

A senior Government source said it was thought Tarrant, who was not on a 'watch list', "transited" through the UK and stayed "for a few weeks".

Security services around the world were building a detailed picture of Tarrant's extensive travels to battlegrounds and cities connected to wars between Christians and Muslims dating back hundreds of years, the Telegraph reported.

The Australian used money left him after his father's death in 2010 and funds raised from investing in online currencies to pay for lengthy trips.

The Mayor of London Sadiq Khan was named in Brenton Tarrant's manifesto. Photo / 123rf
The Mayor of London Sadiq Khan was named in Brenton Tarrant's manifesto. Photo / 123rf

In the summer of 2017 he hired a rental car in eastern France visiting war graves before apparently heading to the UK, possibly by ferry.

French intelligence services are trying to establish whether he contacted any French extremists. His trip coincided with the presidential election when far right politician Marine Le Pen at one point looked likely to win.

Meanwhile, Turkish authorities are investigating the possibility Tarrant had planned a terrorist attack or assassination during two separate visits to Turkey.

In March 2016, he spent four days there before returning from September 13 to October 25. It is thought he had travelled on to Europe, Asia and Africa.

Security cameras captured him arriving at Istanbul's Ataturk Airport. Three Turkish citizens were injured in Friday's attack.

Last November, he flew from Dubai to Bulgaria and spent a week driving a rented car around more than a dozen Balkan cities, including Sofia and Plovdiv, and battle sites where Christians fought the Ottoman army. It is understood he also travelled on to Hungary.

Mladen Marinov, the Bulgarian interior minister, said it was investigating the purpose of his visit.

In 2016 and possibly again the following year, he visited the Balkans touring Serbia, Montenegro and Bosnia.

Croatian police confirmed he had visited their country in December 2016 and January 2017.

He also toured Spain and then Portugal, possibly to Tomar, a former stronghold of the Knights Templar, from the Crusades. He is known to have visited Iceland, Poland, Argentina.

New Zealand is part of a post-World War 2 intelligence-sharing group known as the Five Eyes. It includes Australia, Canada, New Zealand, the United Kingdom and the United States.

A spokeswoman for the New Zealand Intelligence Community said relationships with foreign counterparts across the world were being drawn on as part of its investigation.

The assistance sought and received extended beyond the Five Eyes partners.

"This is the priority focus for the agency and this includes drawing on all international partnerships."

- additional reporting: Daily Telegraph UK