Abd Alkader Habak was busy doing his job, taking photographs, when the explosion happened.
The Syrian opposition photographer and activist was briefly knocked out by the blast, in the outskirts of Aleppo, but quickly got up to find carnage around him.
He decided not to take any more pictures, putting down his camera to help the wounded instead.
The first child he found lying on the floor was dead. The second was alive but barely breathing.
He picked him up and began to run towards safety.
The picture of Mr Habak clutching the boy, aged around seven years old, has been shared on social media tens of thousands times since they were published after Saturday's attack. The particular photograph cutting through the many images of war coming out of Syria.
"This child was firmly holding my hand and looking at me," Mr Habak, 24, told CNN.
"The scene was horrible - especially seeing children wailing and dying in front of you," he said. "So I decided along with my colleagues that we'd put our cameras aside and start rescuing injured people."
The bomb had hit a convoy of residents, who had hours earlier been evacuated from besieged towns elsewhere in Syria.
Witnesses say a man lured children over to a car, where he was handing out crisps and other food. Having lived under siege for more than two years, it was the first treat many of them had had for a while.
After dozens had gathered, there was a large blast.
More than 120 people were killed, including 80 children and 13 women, most of whom Shia from the rebel-besieged towns of Foua and Kafarya, who had been evacuated to what they thought was safety.
Mr Habak put the young boy in the ambulance but does not know what happened to him next.
He then ran back towards the explosion site. Finding another dead child, he collapses to his knees next to the body, overwhelmed.
"I was overcome with emotion," he said "What I and my colleagues witnessed is indescribable."