‘Watch where you’re walking,’ she said, without looking round.

‘Yes, all right.’

She looked up then. ‘Oh. Hi, Diane. How are you doing?’

‘Fine.’

‘Making progress on the enquiry?’

‘Yes, thanks.’

‘There’ll be some publicity on this one, I suppose. There was a TV news van outside earlier. I don’t know what they were filming.’

‘They can film what they like. There’s nothing for them to see.’

She was aware of Petty watching her as she moved around the room. But after a moment, Fry became focused again. She was noticing all the changes that were taking place in the house – the plastic sheeting, the evidence containers, the yellow markers and flags that decorated the carpet, creating a bizarre new pattern on what had once been Lindsay Mullen’s cream Wilton.

‘Actually, I heard you weren’t getting on well with Quinton Downie,’ said Petty.

Fry turned. ‘Where did you hear that?’

‘People talk. Even firefighters have ears, you know. Otherwise, their helmets would fall off.’

‘Very funny.’

Petty looked up at her from her position crouched over a stepping plate. ‘Downie is very well respected in his field. He lectures regularly at Centrex.’

But Fry wasn’t impressed by the mention of the police training centre. ‘That doesn’t mean he has any right to lecture me.’

Downie was in the sitting room packing his equipment away. He looked satisfied with his efforts, reminding Fry of the fire service dog, the chocolate Labrador. He wasn’t quite wagging his tail, but it was a close-run thing.

‘Liquid accelerants are volatile, so it’s good that we collected debris samples early,’ he said when Fry entered. ‘Arsonists tend to use petrol products, because they’re easy to obtain and have a low flashpoint. But petrol has rather a narrow flammability range – it stops burning when the oxygen level is reduced. Hydrogen and acetylene are far more dangerous.’

‘The accelerant in this case could have been a butane-based lighter fluid.’

‘Butane? Well, the flashpoint is about the same as petrol, well below ambient temperature.’ Downie looked around the sitting room. ‘In fact, you’re lucky we’re not looking at radiation-induced flashover.’

‘What?’

‘In a closed room like this, there normally isn’t sufficient ventilation for unlimited burning. In fact, if it had been a bit more airtight, the fire might have gone out. But there was just a little bit of ventilation, and that made it worse. The room was pretty cluttered, items of furniture pushed close together, flammable materials on the floor. In conditions like this, flashover can happen very quickly. If you’re there to see it happen, it’s quite dramatic.’

‘OK, I believe you.’