We don't know what was going through the head of the young killer as he came up behind Jodie Chesney, 17, in a park in East London on Friday night.
We don't know why he stabbed her in the back. It is not clear whether he even knew who the talented and popular student from Havering Sixth Form College was.
It may have been a case of mistaken identity, or he simply may not have cared.
There are some who think the murder was a barbaric gang-inspired initiation ritual.
Beyond the barest details, I am afraid we do not yet know what he looked like, let alone who he was or where he came from. But there is one thing I can tell you about him with absolute certainty.
He thought he could get away with it. He thought he could carry a knife in a public place — with murderous intent — and that there was nothing the rest of us could do.
That is the arrogance feeding the knife crime epidemic.
That is the contemptuous mindset we must change.
We need to take that killer's complacent assumptions about the risk he runs by carrying a knife, and turn those calculations upside down.
We need to come down so hard on kids who carry knives, and above all on gang leaders, that they no longer regard having a knife as cool. We need to flip the switch in their heads so that they stop thinking of it as macho or daring.
We need them to understand that to carry a killer knife is a sign of sheer bone-headed stupidity; not just because they are endangering themselves as well as others, but also because they might end up in jail.
That is the way to fix knife crime — and we have done it before.
Yesterday's Mail front page, carrying the photos of Jodie and 26 other teenagers who have been stabbed to death over the past 12 months, was heart-rending.
I know from experience that all sorts of explanations will be offered for the current plague, and all sorts of solutions.
Some criminologists will talk of postcode wars between drugs gangs. Some will blame deep problems in society: family breakdown, the absence of male role models, and so on.
It is often argued that gang culture gives kids a pseudo-familial structure in which they will find things they crave: authority, boundaries, respect, self-esteem and, yes, love.
All these points have strength — some more than others — and yet they do not fully explain the contagion of knife crime across the UK.
It is true that we need all kinds of interventions to help get these kids' lives back on track, and to deter them from getting involved with gangs.
But if we simply address this epidemic like sociologists or outreach workers, we will not solve the problem and we will not be truly helping these kids. We also need to be tough.
I know this because, 11 or 12 years ago, our capital city was in the grip of an almost identical epidemic. People were horrified at the loss of young life and the violence they could see on their street corners.
I contested the 2008 mayoral election on a simple pledge: to get knife crime down.
I have to tell you that for the first six months — perhaps longer — I thought we would fail. It was an utterly hellish time. I would lie awake praying that the following day would not see another stabbing.
Week after week the toll would mount. We tried everything the experts recommended.
We held endless conferences in City Hall. We had mentoring schemes; we boosted apprenticeships; we tried to cut recidivism for young offenders with a special wing at Feltham Young Offenders Institution.
We funded all manner of youth groups and projects that claimed — some more plausibly than others — to be able to divert kids from gangs.
I am not saying that this campaign was pointless. Far from it. To tackle the temptation to join gangs, it is vital that young people can see a better path for their lives.
But there was also one thing we did that I think helped to reduce the stabbings.
We substantially changed the calculation of risk in the mind of any kid setting out from home with a knife.
We launched Operation Blunt Two — a massive programme of stop and search. In its first year, this took about 10,000 knives off the streets.
Naturally there were protests from pressure groups. Some suggested that stop and search was discriminatory and heavy-handed. We ignored these voices. It worked.
Knife crime began to fall. Serious violence fell dramatically. By the end of my eight-year term, we had cut the London murder rate by 50 per cent and radically reduced the number of teenage stab victims.
For year after year — and this is one of the few crime statistics you really cannot fudge — we had fewer than 100 murders in a city of almost nine million.
It is sad to see these numbers have climbed back up so far. The figures this year are grim.
That does not mean London is now more dangerous than New York (far from it), and I am sure we can collectively beat the problem again.
But it needs energy, and grip — neither of which seems to emanate from City Hall at present — and someone who is willing to take on the politically correct squeamishness surrounding stop and search.
It turned out to be a very grave mistake to tell the police to reduce stop and search in 2014. These searches are an essential tool of big-city crime fighting.
Of course, they must be done sensitively, and in accordance with the law. But if and when our generally superb police officers have reason to suspect someone is carrying a bladed weapon, they must feel free to stop and search that person.
And in so doing — knowing their actions may be resented — they must feel that the system will back them up. They must believe the politicians are 1,000 per cent behind them.
That is the message the police need to hear now, from the very top.
Yes, it is important to have enough officers on the street, and we made a point of keeping London numbers at or near 32,000.
But it is also a question of what you empower those police to do; what encouragement and what lead you give them.
And if they feel they have no ‘top cover’ — no real political lead — then they will not perform the job of stop and search with the same sense of determination.
There was no doubt that by far the most frequently searched group were young black males. Yet the biggest supporters of stop and search were their own parents, who could see that carrying a knife was disastrous and life-threatening for the kids themselves.
I would not for a moment claim that stop and search is the panacea. As Home Secretary Sajid Javid has said, it is not the only solution. But it is a vital part of the mix.
Now is the time to step it up — responsibly but vigorously.
I met Jodie Chesney in March 2016. Then aged 14, she was in the Girl Scouts and went on to become an ambassador for the Scout Movement.
With so much before her, it is utterly senseless that her life has been cut short.
For her sake, and for the sake of all those whose lives are blighted by knife crime, we need to change the odds in the minds of kids who carry knives. And we can.