In the run-up to leaving London, Harry’s two best mates joked darkly about American high-school shootings. One even wrote mind the bullets in his leaving card and drew a stick man letting rip with an Uzi. Now the joke seemed thin.

‘Smoke’s from over by the wood shop,’ someone behind Harry noted as a guidance counsellor urged teens not to shove at the gate. ‘My dad’s a carpenter. One place he worked, there was a spark in the dust extraction and the whole joint went boom.’

‘Where you going, Harry Potter?’ Lupita from Harry’s homeroom spat, as he cut off the path. ‘Ain’t no other gate up there.’

First name Harry, a black mop and an English accent made the nickname inevitable. Even his home-room teacher used it.

Vegas didn’t get a lot of rain, but baked-hard ground meant flash floods when a storm hit. The mesh fence around Rock Spring’s perimeter ran parallel to a concrete drainage channel, eight feet wide and half as deep. Harry stepped down into the basin, brushing weeds growing through cracks as he started a crouched jog towards the smoke.

He glanced back, but fellow evacuees saw nothing past backs of heads and shuffling limbs. The drain’s sides were graffitied, the base littered with occasional pyres made from melted nylon backpacks and black-edged textbooks. These had been squirted with lighter fuel and burnt up by college-bound seniors before summer break.

Harry ducked instinctively as an ambulance skimmed the access road across the fence, lights flashing but siren off. It turned through a set of vehicle gates eighty yards ahead. The storm drain went under this access road, but the thought of snakes in the dark sent a chill down Harry’s spine, so instead of charging through he lay against the gently angled wall, checking the scene as the sun cooked the back of his neck.

Smoke had been tamped by a fire crew, and puddled hose water was evaporating into a rainbow haze. This part of the school was single-storey classrooms, with a taller main assembly hall and lunch room behind. Shatterproof panes had twisted out from their frames, and aluminium roof sheets jutted into the air.

But Harry sensed calm. Two relaxed cops guarded the school’s service entrance as a lunch lady in kitchen whites led a fire officer round the edge of the building, seeking a shut-off valve. Harry cupped one ear and listened to a police lieutenant briefing the freshly arrived ambulance crew.

‘Some kind of explosive. Got the area cleared out and locked down, but don’t hang around inside. We can’t be certain it’s the only device until there’s been a full search.’