Mourners light candles outside the Al Noor mosque in Christchurch, New Zealand, on March 18, three days after the nation's deadliest shooting in modern history. (Vincent Thian/AP)

Shortly after a gunman stormed two mosques in Christchurch, New Zealand in March, killing 51 people, Prime Minister Jacinda Ardern vowed she would never speak the suspect’s name ever again.

“He sought many things from his act of terror, but one was notoriety,” she said. “And that is why you will never hear me mention his name.”

But an apparent lapse in correction facility protocol allowed the suspect, Brenton Tarrant, 28, to send a six-page handwritten letter from prison that has appeared on the online messaging board 4chan. It was addressed to a person he refers to as “Alan.” The recipient of the letter appears to be based in Russia, the Associated Press reported.

The letter describes a trip that Tarrant said he took to Russia a few years ago and warns of a “great conflict” in the future.

He wrote in the letter that he “cannot go into any great detail about regrets or feeling as the guards will confiscate my letter” and could use it as evidence against him. “But I can tell you I have no concern about myself and I only worry for Europe’s future,” he said.

He also thanked the recipient for apparently sending him two postage stamps, describing them as “the only two pieces of color in my otherwise grey cell."

“I will have to hide them from the guards,” he added.

David Bennett, a spokesman for New Zealand’s opposition, said in a statement that the letter also included “a call to action,” saying that “New Zealanders will be horrified” Tarrant was able to send it from behind bars.

The letter was posted to 4chan — an unregulated online forum popular among white supremacists — in the aftermath of other attacks where alleged perpetrators have pointed to Tarrant as inspiration for their own violence.

A suspect in an attack on a Norwegian mosque this past week had posted on social media that he was “chosen” by the accused Christchurch gunman, whom he refers to as “Saint Tarrant,” the Guardian reported.

Ardern condemned that the letter was allowed to be sent, saying Tarrant is someone “who has a very specific goal in mind in terms of sharing his propaganda so we should have been prepared for that.”

“I think every New Zealander would have an expectation that this individual should not be able to share his hateful message from behind bars,” she said.

New Zealand’s Correction Department said it acknowledged that “this letter should have been withheld.”

“We have made changes to the management of this prisoner’s mail to ensure that our robust processes are as effective as we need them to be,” the department said, according to the Associated Press.

Corrections Minister Kelvin Davis also acknowledged that the letter should not have been mailed but said the New Zealand prison system has “never had to manage a prisoner like this before.”

Before the March attack in Christchurch, Tarrant published a lengthy manifesto deriding immigrants and espousing white supremacist views on 8chan, another messaging board that is widely viewed as a more extreme version of 4chan.

Tarrant will appear in court in May 2020 to face 92 charges, including 51 counts of murder, 40 counts of attempted murder and a terrorism charge. He has pleaded not guilty.

Read more:

‘You will never hear me mention his name’: New Zealand’s Ardern vows to deny accused shooter notoriety

Why won’t the U.S. change its gun laws? New Zealand’s Jacinda Ardern says: ‘I do not understand.’

Why New Zealand can do what the U.S. hasn’t been able to: Change gun laws in the face of tragedy