(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.

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