‘All right. I’ll come back shortly. If you want to finish in here, let yourselves out and I’ll close the building later.’

She took a key from her pocket, an ordinary key, the sort used to open house doors. She slid it into a recessed lock on the side frame of the third cabinet, and the front screen of the cabinet opened silently. She passed the key to me.

‘The important thing to remember is that the front screen of this cabinet will close automatically after three minutes. It’s designed to do that, but the hydraulics are powerful. Be careful not to have your hand or anything else in the gap. It’s safe only to look. When the screen closes you can open it again as many times as you like with the key.’

Then she hurried away.

Spoder walked across to the door of the building and made sure it could still be opened from inside. I would not have checked that myself, but I was glad he did. He went to stand beneath one of the wall lights and pulled some papers from his file.

‘I think I understand how Antterland was killed, sir,’ he said. ‘There’s no mystery about the actual cause of death. His assailant shot him using a semi-automatic pistol, a handgun. The bullet went into his chest, close to his heart, and death would have followed almost immediately, within ten to fifteen seconds. There’s a full autopsy report here. The weapon was found: the killer had cleaned off all fingerprints, and left it behind in here when he was eventually released from the building.’ He licked his finger and thumb, and pulled another sheet of paper from the file. ‘Ballistic evidence established beyond doubt that the bullet had been fired from the gun found in here.’

‘It was a simple shooting?’ I said. ‘You made me think it was more complex than that. And what do you mean, the killer was eventually released?’

‘The shooting was the cause of death, but there are extra features which make this murder exceptional. The killer having to be released from a locked room is part of that.’

‘So what else haven’t you told me?’