(CNN) - New Zealand's prison authority has acknowledged it made a mistake in allowing the alleged Christchurch mosque attacker to send a letter, which then surfaced on 4chan, an online message board known for hosting the views of white supremacists.
In a statement released Wednesday, New Zealand's Department of Corrections admitted that the message, written by Brenton Tarrant from his prison cell, should have been withheld.
Most of the six-page screed appeared to be mundane, but the 28-year-old took the opportunity to reassert his anti-immigrant views.
"I have no concern about myself and I only worry for Europe's future," Tarrant wrote. "I only hope our people can survive the coming demographic shift."
The letter was sent to an address in Russia, a country which Tarrant also wrote at length about, before it was posted to 4chan.
The missive's publication comes at a time of increased scrutiny of online messaging boards -- where alleged killers from California to El Paso have posted their hate-filled manifestos.
'Should have been withheld'
New Zealand's Department of Corrections said inmates were legally entitled to send and receive mail but that the prison director may withhold a prisoner's mail in a "very limited range of circumstances."
A spokesperson told CNN that in this instance, this letter should have been withheld.
"On review, we acknowledge that this letter should have been withheld," the spokesperson said. "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."
Tarrant is accused of carrying out the March 15 attacks on two Christchurch mosques, in which 51 people were killed. It was New Zealand's worst mass shooting in modern history.
He is expected to stand trial next year after pleading not guilty in June to 51 counts of murder, 40 charges of attempted murder, and one charge under the Terrorism Suppression Act, the first time such a charge has been laid inside the country.