The hearing had a starkly different tone to Mr. Tarrant’s previous court dates, when the public gallery had usually been packed with victims of the shootings and relatives of those who died, with no seats remaining for interested bystanders.

On Thursday, reporters outnumbered members of the local Muslim community in court. About 200 people from the two Christchurch mosques that were attacked on March 15 have traveled to Mecca for the hajj pilgrimage, as guests of King Salman of Saudi Arabia. Muslims are required to make the pilgrimage at least once in their lifetimes if their physical and financial circumstances permit, but Muslims in far-flung New Zealand said they had previously been constrained from doing so because they could not afford the travel costs.

Kelvin Davis, New Zealand’s corrections minister, told Radio New Zealand on Thursday that the letter that appeared online this week was one of five Mr. Tarrant had been allowed to send. He had been barred from posting two others.

Mr. Davis said the suspect had received “a couple of dozen” letters from around the world, some of which had been kept from him. New Zealand law allows prisoners to receive mail unless there is good reason they should not.

The corrections agency has issued an apology and said it would bar Mr. Tarrant from sending letters until it can be sure it has proper processes in place to check them.

Photos of the letter circulating on the internet showed plain lined notepaper covered in childlike handwriting, the corners of each page carefully numbered.

In it, Mr. Tarrant begins with nonchalant recollections about a 2015 trip to Russia, where his correspondent apparently lives, before veering into his racist influences and ending with a call to violence.