He sighed and stood up. Together, they made their way out of the house and into a side passage near the garage. Brian Mullen’s car still stood on the drive. It was a red Citroën, almost the same colour as the fire appliances that had surrounded it on Sunday night.

‘OK,’ sighed Downie. ‘Look, you have plumes of soot deposited on the exterior wall by smoke emitted from the window. But the broken glass on the ground beneath the window is unsooted. So, we can conclude that the fire didn’t touch this glass.’

‘Yes, I see.’

‘Even from here, I can see tool marks on the window frame,’ said Downie. ‘You might care to check whether the firefighters obtained entry this way.’

‘They didn’t. They came in through the doors.’

‘Right.’ Downie turned to look at her. ‘Pity about the shoe impressions, though.’

‘What shoe impressions?’

‘Precisely.’

Fry looked at the ground where they were standing. It was a muddy mess, covered in crushed vegetation and trampled by size ten boots.

‘Shit.’

Downie shrugged. ‘Think yourself lucky to get this much. The site of any fire is a challenge to the principles of crime-scene management.’

‘If the lab finds butane in your samples, it won’t be up to me any longer anyway, lucky or otherwise,’ said Fry. ‘It becomes a murder enquiry.’

‘I know, I know.’

Fry felt herself getting angry. ‘Three people died in this fire. The evidence mustn’t be compromised.’

‘I can assure you, Sergeant Fry, everything will be done by the book.’

Fry looked at the rest of the houses in the street. A few neighbours were clustered outside the cordon. By the book, eh? That was all she needed, some civilian lecturing her about procedure. She knew what ‘by the book’ meant.

She also knew the principles Downie was referring to: protect, record and recover. Crime-scene examiners said that contamination only really occurred after the scene had been preserved. Anything before that was normal procedure.

But in this case, procedures had involved smashing down the doors and flooding the place with water, then sending in firefighters in big boots to trample the sodden evidence. Well, the principles still applied. As long as compromises were recorded and reasons given.

‘By the way,’ called Downie, passing the RV point on the way to his vehicle, ‘the usual advice is not to fit a smoke alarm in the kitchen. Steam and cooking fumes can set it off too easily. For a two-storey house like this, the bottom of the staircase is the best location, with a second one on the landing as an extra precaution.’

‘I’ll be sure to let Mr Mullen know,’ said Fry.

‘Who?’