Update: The article below was written in the hours following the attack, after police had arrested four people. Police has since said only one of those four was involved in the attack. The questions raised by the attack remain relevant.
How did we miss this? The little we know asks questions of those who would keep us safe.
We need answers.
At first reports, New Zealand appeared to have a terrorist in the style of Anders Behring Breivik, the far-right terrorist who killed 77 people in Norway in 2011.
Breivik was a "lone wolf" - someone who planned alone and carried out his attack alone.
This killer was something different. There are signs which point to an ugly rotten core in our society which could have been identified earlier.
In his online manifesto, he said he had recently come to New Zealand from Australia to plan an attack.
When he settled in, he decided New Zealand was the place to carry it out.
• Live: Massacre at the mosques - gunmen kill up to 30 people
• Christchurch mosque shooting: What you need to know
• Mosque shooting: Dad went in for prayers and never came out
• All Black Kieran Read locked in, waiting for his children during massacre
Something happened in Christchurch which changed his views. In his manifesto, he said "an attack in New Zealand would bring to attention the truth of the assault on our civilisation, that nowhere in the world was safe, the invaders were in all of our lands".
That sounds like he's been radicalised. And the fact the attack happened here suggests whatever influence there was on his thinking, it was domestic in origin. He found like minds.
Then there's the firearms.
He had multiple firearms and, of those he did have, witnesses report the shots sound like "firecrackers".
While some semi-automatic rifles can be bought with a basic gun licence, repetitive semi-automatic weapons with extended magazine capability of the sort witnesses describe need a special category of gun licence.
It is meant to involve extensive police checks and background inquiries of the prospective owner.
It seems unlikely a recent immigrant — Australian or wherever — who came to New Zealand and sought out the most restricted licence would not raise a flag.
It seems far more likely the weapons were supplied by people already here.
As information emerged today, we learned four people had been arrested - three men and a woman.
Not only did they have extensive weaponry, they also managed to plant improvised explosive devices across the city.
It's one thing to fly beneath the radar as a lone wolf.
It's a completely different proposition to join and develop a functional and organised terrorist cell which can deliver compelling rhetoric to new recruits and then provide weaponry, and knowledge, to carry out an attack such as today's.
Cells operating as a group require communication and co-ordination which increases the number of points at which authorities can notice and disrupt their plans.
We have a number of security agencies in New Zealand which will face serious questions.
The easiest to contemplate is the firearms. Police Minister Stuart Nash is looking at firearms law now. He needs to look harder.
In Whangārei, we had murder committed by an angry man who bought weapons illegally with extraordinary ease. In Rotorua, we had a killer convicted on forensic evidence which owed little to police systems but to exceptional diligence by a lone police office and ground-breaking Australian forensic work.
That's only the firearms.
The real danger are the people who choose to carry them and commit acts such as we have seen today.
The police force, which expends huge effort gathering and ordering intelligence on gangs, will need to consider whether it committed sufficient resource towards the increasingly polarised, hate-filled groups which have sprung up across Western nations.
Gangs largely prey upon themselves. Groups with extreme views prey upon the rest of us.
These groups have been responsible for a number of massacres in Western countries, which should have tripped warning bells.
There are hard questions for the NZ Security Intelligence Service. It has - like its Western counterparts - a strong focus on potential threats in the Islamic community.
Has it dedicated the same effort to other parts of society? It certainly used to. Pre-September 11 NZSIS tasking files pay huge attention to neo-Nazi, far right groups.
The NZSIS - and its electronic counterpart, the Government Communications Security Bureau - have more funding than ever, and almost double the staff numbers they had six years ago.
They also now have the most powerful legislation they have ever had.
This attack isn't a call for new powers or greater funding. The spies have all they need in a society such as ours - even with the shocks of today.
Instead, we need to check how they have grown into that accelerated growth in funding and broader legislation.
It was only last year NZSIS director general Rebecca Kitteridge told the NZ Herald it had struggled to match its improved capability against its need.
She had told the former National Party spy minister Chris Finlayson in 2015 "in the context of the threat environment we are facing, the NZSIS capabilities will continue to be less than the demand on our services".
"We will need to continue making difficult prioritisation decisions about which targets we investigate [and for how long] and which we do not."
A Royal Commission - in public, with open evidence - needs to ask those questions. The country deserves evidence and answers. And let's not hear overblown claims of "classified information".
It also needs to check when the Inspector-General of Intelligence and Security should have funding and staff to match the agencies her office oversees.
There's a considerable gulf between what she has to work with and the work she needs to do.
There's nothing which sharpens the focus of a security service - unless it's an attack of this sort - than an oversight agency working hard to make sure it is doing its job properly.
NZ Herald journalist David Fisher is a member of a Reference Group formed by the Inspector General of Intelligence and Security intended to hear views on developments possibly relevant to the work of the oversight office. The group has a one-way function in offering views to the IGIS. It receives no classified or special information from the IGIS or the intelligence community. The information in this story was not sourced from Reference Group discussions.