The accused Christchurch gunman appears to have been allowed to send a potential "call to action" to his global support base of white supremacists.
It appears the letter from his jail cell was not stopped by Corrections despite a spate of copycat killings around the world - and is now being circulated on a notorious website.
This is potentially the first communication the accused Christchurch gunman is believed to have had with the outside world - a letter, that appears to have been sent from Auckland Prison at Paremoremo.
Newshub has been unable to confirm the origins of the letter.
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It was posted on 4chan on Tuesday, a message board infamous for graphic and violent content, with a user saying: "A couple of months ago I sent a letter to Brenton Tarrant and today I finally got a response."
The letter starts: "Hello Alan, Thank You greatly for sending the letter." Much of the letter appears to be innocuous.
But it ends with what is effectively a call to action for like-minded people, that we have decided not to broadcast. It is signed off by the alleged gunman.
Ben Elley, an expert in extremism online, says the final line was enough to provoke a reaction among the far right.
"I think there certainly are people out there in those communities who will read it as a call to arms," he told Newshub.
"Whether that's a call to commit other acts of terror or not isn't clear - people will interpret it in their own way."
The alleged gunman is revered in these extremist online communities as a "Saint".
"It is certainly very threatening," said Elley.
On the day of the mosque attacks, a manifesto was posted on a similar site called 8chan. But after that site was also used by a shooter at a synagogue in California, and then the shooter in El Paso - it was shut down.
Up until recently, much of the white supremacist extremism has happened on 8chan. But 4chan is still going.
"In many ways it's just a flag held up for them to say 'yes, I am ok, I am doing fine,'" said Elley.
A former inmate and people who correspond with Auckland Prison a lot have confirmed the stamp belongs to the jail.
And a handwriting expert says the letter was probably written by the alleged gunman.
Late on Wednesday, the Government confirmed the letter is real.
The Corrections Minister Kelvin Davis says he is angry at his department for letting the letter get out.
It is very important New Zealand see this in context - there is a fear in nearly every country in the western world of a copycat attack from supporters of the alleged gunman.
Somehow, a message has been allowed to get out of a New Zealand prison which has potential to inspire them. That is a big concern for global security around the world and it has the potential to be internationally embarrassing for New Zealand.
Newshub has been investigating 8chan for months now and the correspondence on the site references "bombarding" the prison with calls and letters as a tactic to reach Brenton Tarrant.
Users also say things like "They're going to be filtering as much as they can so a wide range of letters will make it more likely something will get through."
In this case, it turns out their tactics worked.