Schemm stirred himself to offer up some facts and figures. ‘I think I can be of some assistance here, Herr Six,’ he said, delving into a briefcase that lay by his feet, and producing a buff-coloured file which he laid on the rug between the two sofas. ‘I have here the last insurance valuations, as well as some photographs.’ He lifted a sheet of paper and read off the bottom-line figure with no more expression than if it had been the amount of his monthly newspaper account. ‘Seven hundred and fifty thousand Reichsmarks.’ I let out an involuntary whistle. Schemm winced at that, and handed me some photographs. I had seen bigger stones, but only in photographs of the pyramids. Six took over with a description of their history.

‘In 1925 the world jewel market was flooded with gems sold by Russian exiles or put on sale by the Bolsheviks, who had discovered a treasure trove walled up in the palace of Prince Youssoupov, husband to the niece of the Tsar. I acquired several pieces in Switzerland that same year: a brooch, a bracelet and, most precious of all, a diamond collet necklace consisting of twenty brilliants. It was made by Cartier and weighs over one hundred carats. It goes without saying, Herr Gunther, that it will not be easy to dispose of such a piece.’

‘No, indeed.’ It might seem cynical of me, but the sentimental value of the jewels was now looking quite insignificant beside their monetary value. ‘Tell me about the safe.’

‘I paid for it,’ said Six. ‘Just as I paid for the house. Paul didn’t have a great deal of money. When Grete’s mother died I gave her the jewels, and at the same time I had a safe installed so that she could keep them there when they weren’t in the vault at the bank.’

‘So she had been wearing them quite recently?’

‘Yes. She accompanied my wife and myself to a ball just a few nights before she was killed.’

‘What kind of safe was it?’

‘A Stockinger. Wall-mounted, combination lock.’

‘And who knew the combination?’

‘My daughter, and Paul, of course. They had no secrets from each other, and I believe he kept certain papers to do with his work there.’

‘Nobody else?’

‘No. Not even me.’

‘Do you know how the safe was opened, if there were any explosives used?’

‘I believe there were no explosives used.’

‘A nutcracker then.’

‘How’s that?’

‘A professional safe-cracker. Mind you, it would have to be someone very good to puzzle it.’ Six leaned forward on the sofa.

‘Perhaps,’ he said, ‘the thief forced Grete or Paul to open it, then ordered them back to bed, where he shot them both. And afterwards he set fire to the house in order to cover his tracks - throw the police off the scent.’