In the professor’s study, the atmosphere was cool. No drinks offered, no pleasantries, no invitation to take the most comfortable seat. Fry pulled a chair close to Robertson’s desk and rested her elbows on it, forcing the professor to lean backwards to avoid appearing too confrontational.

‘In your discussions with Detective Constable Cooper, you’ve been very helpful over the past few days, sir,’ she said.

‘I’m gratified to hear it. I do my humble best.’

He looked towards Cooper, beginning to form a smile. But Fry wasn’t going to let his attention drift, or the mood soften.

‘You might be able to assist us even more, sir.’

‘Oh?’

‘Looking at the facts we’ve gathered in our enquiries so far, it seems rather strange that you’ve been able to cast light on almost every aspect.’

‘Strange? Why should it be strange? This is my area of expertise, Sergeant. My role is to cast light where it’s needed.’

He was trying to keep his tone breezy, but Fry knew she’d needled him.

‘Yes, sir. But I find it particularly interesting that you’ve several times managed to cast your light before you were asked the appropriate question.’

‘I have no idea what you mean.’

‘The sarcophagus, for example. The “flesh eater”. You were very keen to tell us all about that. So keen, in fact, that you mentioned it to DC Cooper before he had any idea of the relevance himself.’

Robertson tried for another conspiratorial smile with Cooper, but it wasn’t working. For once, Cooper was obeying the instructions she’d given him and was staying detached.

‘The same applies to some of the other references in the messages,’ said Fry.

‘Ah, the messages.’

Fry felt a surge of surprise and excitement. ‘You admit that you know about the messages?’

‘I know nothing about any messages,’ said Robertson. ‘Except that your colleague mentioned them to me yesterday.’

And now the professor smiled. Fry heard a squeak of leather, and sensed Cooper shifting uneasily in his chair. So she knew it was true. But she had to press on.

‘Professor Robertson, I think you already knew about the calls that have been made to us during the past week. I believe you’re familiar with their exact content. Do you own a device called a voice changer?’

‘Sergeant, this is quite intolerable. When I agreed to put my time and expertise at the disposal of the police, I didn’t expect to be treated in this way.’

‘May I see your car keys, sir?’

She’d struck home this time. Fry saw the professor’s eyes widen and his nostrils flare as his expression tensed. He jerked forward in his chair as if she’d prodded him with a sharp needle.

Fighting to control his anger, he opened a drawer and tossed a ring of keys on to the desk between them. Fry didn’t touch them, but separated the keys one by one with the end of her pen. Robertson’s face suffused with blood as if she’d uttered the most deadly insult.