‘I need you to see what you can get off this,’ said Erika, handing Nils the smaller clear plastic evidence bag containing the memory card. ‘It had been swallowed, but not for long.’

‘Shouldn’t be a problem,’ he said, taking the bag. ‘But first, I need you to see something.’

They followed him back inside and down the cream carpeted hallway, still busy with CSIs in blue overalls, through the open-plan living area and into the utility room that overlooked the back garden. The door was open. They stepped back out into the sunshine. In the distance, a lawnmower whirred.

‘We’ve checked all of the windows in the house. They are a mixture of UPVC plastic and triple glazing that is very hard to access unless you break them. They are all locked from the inside, apart from the bedroom window, which was smashed by the colleague of Jack Hart when she discovered his body,’ said Nils. Erika and Moss followed his gaze, looking up at the broken window at the back of the house. ‘There are no prints or any other signs of forced entry.’

‘What about the front door?’ asked Erika.

‘Locked from the inside with a Yale lock and a dead bolt,’ replied Nils. ‘Which leaves this, the utility room door, which I believe was the point of entry.’

The door was made of stout wood, and painted with deep blue gloss. The handle was of heavy iron, and the sturdy metal key Erika had found on the shelf was in the lock on the inside.

‘It was locked. I had to unlock it when we chased after the photographer,’ said Erika.

‘I’ll get to that in a moment,’ said Nils. He pushed the door closed. ‘If you look very closely here on the outside, there’s a tiny strip of wood at the bottom which has an older coat of paint.’ They crouched down on the grass outside the door and noted the centimetre of pale green running along the bottom of the door frame.

‘A draught-excluding strip had been stuck on with adhesive when the door was green. The draught-excluding strip had recently been removed; we found it behind the washer and the dryer,’ said Nils, opening the door and stepping back into the utility room to retrieve a long, thin strip of rubber from the top of the washing machine. He placed it over the green strip at the bottom of the door and then took it away again. ‘Can you see where it’s been peeled away from the base? It leaves a quarter-inch gap under the door.’

Erika looked at Moss.

‘That doesn’t explain how he got in, unless he’s Flat Stanley,’ said Moss.

‘I’ll show you,’ said Nils. He motioned to one of the fingerprint technicians, who came through from the kitchen with a long piece of wire and a sheet of newspaper. He then closed the door and locked it, leaving them outside in the garden. Nils knelt down, unfolded the double sheet of newspaper and slid it through the quarter-inch gap under the door. He then took the wire, slotted it through the keyhole, and gently pushed and twisted the wire. Erika and Moss watched through the window and saw the key shift, pop out of the lock and land with a clink on the newspaper below. Nils then carefully slid the newspaper out from under the door, bringing the key with it, which he inserted into the lock and opened the door.

‘Voilà!’ he grinned, triumphantly.

They stared at him for a moment. A small drain beside the door gurgled.

‘You’re wasted in forensics. You should have your own magic show,’ said Moss.

‘It’s brilliant, but how do you know this is how he got in?’ asked Erika.

‘We found a piece of broken wire inside the lock, and a small piece of newspaper had caught on the wood under the door,’ said Nils, producing an evidence bag from his pocket with a flourish. It contained a stub of silvery wire and a scrap of torn newspaper.

An image popped into Erika’s head. A bathroom filled with steam. Mark, wearing just a towel around his waist, pressing a similar-sized scrap of toilet tissue onto a shaving cut, a spot of blood soaking through.

The whirr of the lawnmower starting up again jerked Erika back to reality.

‘Were there any prints on the draught-excluding strip?’ Moss was asking. Nils shook his head. ‘If the killer got in using this newspaper-under-the-door trick, how did they get out again – lock the door and leave the key on the shelf?’

‘They didn’t. Just like at Gregory Munro’s house, they could have paid a visit previously. Taken the key, had a copy made, and then replaced it,’ said Moss.

‘It makes sense. It’s a bit of a stretch, but it makes sense. Would it hold up in court, though?’ replied Erika.