New Zealand officials admitted Wednesday they made a mistake by allowing the man accused of killing 51 people at two Christchurch mosques to send a handwritten letter from his prison cell.

The six-page letter from Brenton Tarrant was posted Tuesday on 4chan, a website that has become notorious as a place for white supremacists to post their views. And it comes at a sensitive time, with other alleged killers from El Paso to Norway citing Tarrant as an inspiration.

The letter appears to be written in pencil on a small notepad and is addressed to “Alan” in Russia. Much of it appears to be relatively innocuous, discussing a one-month trip Tarrant says he took to Russia in 2015. But the letter also warns a “great conflict” is coming and uses language that could be construed as a call to arms.

‘This cannot happen again’

Corrections Minister Kelvin Davis said in statement he didn’t believe the prison system should have allowed Tarrant to send the letter.

“I have made myself clear that this cannot happen again.”

But Davis also said all New Zealand prisoners have rights that include the ability to send and receive mail. He said the prison system can withhold correspondence and withheld some other letters Tarrant had attempted to send or receive.

“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.

New Zealand Prime Minister Jacinda Ardern has vowed never to utter Tarrant’s name to deny him the publicity she says he craves, making Tarrant’s letter even more of an embarrassment for the government.

New Zealand Prime Minister Jacinda Ardern, centre, hugs and consoles a woman during a visit to the Kilbirnie Mosque in Wellington in March after the shootings. (TVNZ via The Associated Press)

In the letter, dated July 4, Tarrant thanks Alan for postage stamps he apparently sent, saying they’re the only two pieces of colour in an otherwise grey cell and adds he’ll have to hide them from the guards.

Tarrant cites Plato and other philosophers and writers as inspiration for his views, and says 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.

Opposition spokesman David Bennett said Davis needed to demand immediate answers as to how an inflammatory letter could be sent from inside a maximum-security prison.

“This man is accused of carrying out one of the most heinous crimes in New Zealand history,” Bennett said in a statement. “New Zealanders will be horrified that Corrections allowed him to send a letter which includes a call to action and has subsequently been posted online.”

The Corrections Department, which oversees prisons, said the law only allows a prison director to withhold a prisoner’s mail in a “very limited” range of circumstances.

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

Posted 74-page manisfesto online before shooting

Before the March 15 shootings, Tarrant, a 28-year-old Australian white supremacist, posted a 74-page manifesto on 8chan in which he outlined his views and beliefs that immigrants were invaders who would replace the white race.

The website 8chan, seen as a more radical offshoot of 4chan, was effectively knocked offline this month after two companies cut off vital technical services in response to claims that the gunman who killed 22 people at a Walmart in El Paso, Texas, posted an anti-Latino screed on the site just before the Aug. 3 killings.

Mourners arrive for a burial service of a victim a week after the March mosque shootings in Christchurch. (Vincent Thian/The Associated Press)

Like the Texas gunman, a Norwegian man suspected of killing his stepsister and then storming an Oslo mosque with guns this month is also believed to have found inspiration in Tarrant’s actions.

“I think every New Zealander would have an expectation that this individual should not be able to share his hateful message from behind bars,” Ardern told media on the island nation of Tuvalu, where she is travelling to attend a meeting of Pacific leaders.

It’s not the first misstep by New Zealand authorities in the case. Police initially filed a single representative murder charge against Tarrant, but mistakenly named somebody who was still alive before later amending the charge.

Tarrant has pleaded not guilty to terrorism, murder and attempted murder charges following the mosque attacks. He remains in jail ahead of his trial, which has been scheduled for next May.