New Zealand officials on Wednesday admitted they should not have permitted the man accused of killing 51 people in a massacre at two mosques last year to send a hand-written letter from prison.

The letter penned by Brenton Tarrant was posted this week on the website 4chan, according to The Associated Press, and warned of a “great conflict.”

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The letter, which is addressed to “Alan” in Russia, includes inoffensive details such as those of a monthlong trip to Russia but also includes language that could be interpreted as inciting, according to the AP.

“I have made myself clear that this cannot happen again,” Corrections Minister Kelvin Davis said in a statement about the letter.

While Davis noted that all prisoners in New Zealand have the right to send and receive mail, he noted that the prison housing Tarrant withheld some other letters he tried to send or receive and opened the door to future changes to laws regarding the oversight of prisoners.

“We have never had to manage a prisoner like this before — and I have asked questions around whether our laws are now fit for purpose and asked for advice on what changes we may now need to make,” Davis said, according to the AP.

The Corrections Department also noted that while a prison director can withhold a prisoner’s mail in a “very limited” range of circumstances, it should have prevented Tarrant’s letter from getting out.

“On review, we acknowledge that this letter should have been withheld,” the department said in a statement to the AP. “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.”

Before the March shooting, Tarrant posted a 74-page manifesto on the website 8chan promoting his white supremacist views and casting immigrants as invaders.

Tarrant cited Plato and other philosophers in the letter as inspirations for his views, writing he “cannot go into any great detail about regrets or feelings as the guards will confiscate my letter if I do” and use it as evidence.

The suspect in the recent mass shooting in El Paso, Texas, pointed to Tarrant as an inspiration, as did another shooter at a mosque in Norway.

Tarrant has pleaded not guilty to charges of terrorism, murder and attempted murder. He will remain in jail leading up to his trial, which is scheduled for next May, according to the AP.