At the foot of the stairs, she turned right into the sitting room, keeping carefully to the stepping plates. Judging by what the neighbours reported, the occupants of 32 Darwin Street had been taken by surprise. Six weeks ago, Lindsay Mullen had ordered a new carpet for her lounge. It had a deep, thick pile, and it was a shade of cream Lindsay had always wanted, but her husband insisted wasn’t practical. It would show up the dirt, he’d said. It was a shameful waste of money.

Royal Wilton in chamomile, that was it. According to the uniforms who’d taken initial statements, the lady in the house to the left had heard the entire argument as Brian Mullen left for work one morning.

Fry looked around the wrecked lounge. Mr Mullen had been right. The carpet was black now, and trampled with charred debris. Dozens of boot prints had sunk into two inches of filthy sludge.

The problem was, the bottom edge of the door into the kitchen had sealed so tightly against the new Wilton that there was no gap for air to get through. When the sofa ignited, thick smoke would have filled this room in minutes, choking fumes that seared the lungs and stung the eyes. If only a few wisps of it had seeped under the door into the kitchen, they might have reached the smoke alarm, and the outcome could have been different. Unlike so many others, the Mullens’ alarm had been working, the batteries recently replaced. It just hadn’t detected any smoke until it was too late. Far too late.

‘They never stood a bloody chance, did they?’ said Murfin.

Fry glanced at him. His flippancy was gone, and he was sweating a little despite the draught stirring the curtains behind him. Of course, Gavin was a family man, with children of his own. There were some things that got to you, no matter how hard you tried to keep up the exterior.

‘They say it’s better to die of smoke inhalation than burn to death, anyway,’ she said, though she didn’t expect it to help. Her own flesh still crawled at the thought of the flames.

She looked away from Murfin before he could distract her concentration. This room had been packed with plastics, too – TV, video recorder, racks of CDs and DVDs, boxes full of children’s toys under a shelf in the corner. Most of the toys were just a molten mess now, multi-coloured pools of lava that had run on to the carpet and congealed in the spray from the firemen’s hoses. There were recognizable shapes here and there – the twisted controls of a PlayStation console, the burned edge of a Monopoly game. The head and one arm of a Barbie doll waved from a skin-toned puddle, like someone drowning in a sea of their own flesh. Something scorched and wooden gazed accusingly at her from blackened eyes.