'Metres away, I see a rocket kill one boy, leave his brother unscathed. Terrible fate in action'

Photographer Jeremy Kelly and I were at a polling station in Lashkar Gah to cover Afghanistan's presidential elections, from Helmand province.

People die every day in the insurgency here, but it took the terrifying screech of an indiscriminate rocket and the carnage that followed its impact to show me what "civilian casualties" really means.

I was 60 metres from the crossroads where two young brothers – no older than 12 – were balanced on a bicycle, meandering along the empty streets of the provincial capital.

They were heading towards the river, where there is a small park with swings. Schools were closed for the day because most of them were acting as polling stations. Perhaps, like so many other children, they were going to jump off the rocks and swim in the river.

At first, it sounded like a car crash. The deafening sound of wheels skidding on tarmac followed by a crash that sounded like thunder. But the plume of dark black smoke told me it was something much worse.

I was standing on a concrete terrace, along one side of a football field, which had also been turned into a polling station. The boys were pedalling towards me.

After the blast came silence, for a second or so, until the boy who survived found the breath to wail. A terrible, painful wail.

They were less than 200m from the Emergency Hospital. Police and medics were on the scene in seconds. By the time I'd taken stock and started to move again, the ambulance was already there. Without a stretcher, four men – most of them medics – lifted the limp body into an ambulance. He was wearing a beige shalwar kameez. His bicycle lay abandoned on the road. His sandals were strewn across the tarmac.

What was left of his head hung backwards, smearing blood over a plastic bag in the boot as they struggled to heave him in. The other brother was still wailing.

When he turned away from me, staggering, I saw that his back was smeared with blood and bone and brain.

Less than a minute ago, they were sat just a few inches apart on a bicycle. Now one of them was fatally injured and the other, miraculously, was physically unscathed. I had just seen fate in action.

At first I was too terrified to feel compassion. I'd run out to the place where a rocket had dug into the road and killed a passer-by. The Taleban had warned us there would be more and I was scared for my life.

A dome-shaped piece of bone – unmistakably part of his skull – and raw human flesh marked the spot where they'd fallen.

The remnants of the rocket sat resting on the tarmac. It looked far too still for something that had caused so much damage. The air smelt bitter with explosives.

What was most astonishing was the speed with which everyone else went back to normal. The ambulance left. The road was hosed down, and voters walking past didn't seem to notice.

The rockets kept on falling as we drove around the city. Every time we stopped, I walked close to the walls and looked for possible cover. No one else seemed to notice.

"We are not scared, we are Afghan," said 15-year-old girl Shugufa, who'd managed to vote despite her age.

In more than 40 short interviews yesterday, not a single person even flinched as the rockets continued to land. The rumble of explosives here is normal.

After so many years of war, people seem inured to violence. I hope I never am.

Jerome Starkey has reported for The Scotsman from Afghanistan since October 2007.