In the aftermath of the violent shopping centre siege that left dozens of people dead, many in Kenya and neighbouring Somalia are searching for answers about why and how the attack could happen.
A few months ago, I was sitting in Dr Yaseen Nur's kitchen, above his private hospital in Mogadishu.
Some wounded soldiers had just been brought in to the Somali capital - the survivors of a massive roadside bomb that had gone off on the outskirts of the city.
Dr Nur - a short, talkative man - was taking a break from surgery, sipping tea, and telling me a chilling story about a young relative - a 19-year-old - who had recently arrived on his doorstep unannounced. The young man had grown up in exile in Germany, like so many who have fled abroad to escape Somalia's endless anarchy.
But as Dr Nur put it, the boy had spent too much time in front of his computer reading about jihad. And so he'd come back - without even a suitcase - to join al-Shabab - the Islamist militants who have claimed responsibility for the horrors in Nairobi.
He spent a night here, said the doctor. He barely spoke. Then he vanished. He called us just once to say he was now married, and preparing to fight the infidel.
If you're flying from Mogadishu to Nairobi, the security checks are slow and thorough. All planes make an extra stop in northern Kenya just to search the passengers one more time.
But it is easy enough to drive through the bush, bribe any policemen you stumble across, and vanish into the dusty streets of Eastleigh, the chaotic, thriving Somali neighbourhood in Nairobi.
I used to live in the Kenyan capital - just over the hill from Eastleigh. That was 10 years ago, before the Chinese had started building the giant highways that are slowly unblocking the city's traffic-choked streets.
Nairobi is changing fast. New shopping centres. Rooftop bars serving sushi. Ludicrously high rents.
But it is a place - like much of Africa - that tends to be seen by foreigners in extreme terms. Kenya is either a beach and safari paradise, or somewhere forever teetering on the edge of machete-wielding chaos.
The truth, of course, is more prosaic.
This is a compassionate, chaotic, occasionally violent, exceptionally tolerant nation - a country that has absorbed vast numbers of refugees over the years with barely a shrug.
But two years ago, Kenya's patience with Somalia finally snapped, and it sent troops across the border - hoping to create a buffer zone of stability.
Al-Shabab immediately and repeatedly promised to respond with exactly the sort of carnage that finally engulfed Nairobi's Westgate shopping centre.
The details of the attack and the attackers are still being sifted from the ruins. Was the British widow, Samantha Lewthwaite involved? Was this a Somali revenge operation. Or was it part of a much broader global al-Qaeda agenda?
It was certainly well planned.
You will have heard the stories: Of methodical evil, of children shot at point blank range, of grenades tossed into blood-drenched rooms.
And you may have seen the pictures - of startling, selfless courage. Of unarmed men running into danger, again and again, to coax families literally paralysed with fear, to race to safety.
Nairobi has always felt like a small town. Almost everyone I know here has lost a friend, or had a narrow escape, or has some other visceral personal connection to the horrors of Westgate.
Yusuf Hasan lives just down the road from the shopping centre. He is still in a wheelchair after a grenade was thrown at him last year, possibly by a member of al-Shabab. Yusuf is a Somali Kenyan, and a member of parliament.
"We always go to Westgate on Saturday morning," he told me. But by chance, not this time. Even after the grenade attack I felt safe in this city. But that's changed completely now.
Yusuf feels the Westgate attack has pulled Kenya closer together - united it in grief and outrage - and he thinks that will last.
But in parliament, they're already asking tough questions - about why the security services took hours to respond properly. About corruption and infighting in the intelligence services. About how Kenya's army should fight back and whether it should allow itself to be dragged even deeper into the war in Somalia.
I've not been able to reach Dr Nur this week. His phone rings unanswered.
But I imagine him at his clinic in Mogadishu, waiting for whatever the ambulance brings next, and wondering if his young relative will ever call again.
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