Cordelia took this as a cue to begin gently probing. “When you were backstage on Saturday night, did you see anyone at all going down the corridor toward Music 2?”

“I don’t really remember. I was rushing round so much organizing the choirs and the orchestra. But I would have thought nothing of it even if I had, because the music storeroom is there too, and that’s where the programs were being kept.”

“Did you go to your own room?”

“Several times, but I didn’t stay there for any length of time, as I’ve already told the police. I didn’t pay any attention to the keys, and I didn’t see anyone else go in or out of my room.”

“How many people knew that Lorna was going to be in Music 2 before her performance?”

Margaret thought for a moment. “It was no particular secret,” she replied. “Most of the girls involved in the concert could have worked it out for themselves, since that room had been left free. I think I mentioned in the staffroom that I’d had the fifth form clearing it up on Friday so that it would be tidy for Lorna. I wanted to make sure that everything was just right, so there could be no criticism from her of my preparations. So the answer to your question is that anyone could have known. There’s no way of narrowing it down.”

“I suppose in theory you yourself could have taken the key, gone to Music 2, done what you had to do, and put the key back in the wrong place—deliberately?” Cordelia inquired.

Margaret Macdonald’s hands worked in her lap. There was fear inadequately hidden in her eyes and Lindsay noticed a trace of sweat on her upper lip. “I suppose I could have. But I didn’t. Look, Lorna was the most distinguished pupil I’ve ever taught; she was about to give a concert in aid of the school. Why should I have wanted to kill her? This is nonsense!”

There was a pause. Lindsay could sense that when it came to questioning her former teacher, Cordelia had no killer instinct. But she was loath to take the hard line of questioning herself. For once she felt unhappy at subjecting someone to her professional probing. She didn’t want to strip this woman’s defenses down because she was sure she was a sister of sorts. She should be offering her support, not giving her a hard time. Then she thought about Paddy, “I know you taught Lorna, and obviously she owed a lot to you,” she said decisively. “Did you remain close?”

Margaret studied the carpet as if inspecting it for some clue as to how she should respond. She said quietly, “Not since she left school. We exchanged Christmas cards, that was all. She had a very hectic life.”

“I’m surprised you didn’t stay more closely in touch.”