Monk understood completely. A man of Havilland's seniority and experience losing his nerve would be enough to create hysteria that could bring about the precise disaster he was afraid of. At the very least it would disrupt work, perhaps for days, and consequently the next project would be sure to go to a rival.

"Did you suspect it could be deliberate?" he asked.

Sixsmith was momentarily puzzled. "Deliberate weakness? He'd make himself unemployable anywhere else, which would be stupid. Why would any man do that? And he and both the Argyll brothers were friends. Family, in fact."

"I meant sabotage, for a suitable reward," Monk explained, but it sounded ugly as he said it, and he saw the revulsion in Sixsmith's face.

"From another company?" Sixsmith's lips curled. "If you'd known Havilland, you wouldn't even ask. He might have hid his weaknesses, and he might even have been something of a coward, but he was absolutely honest. He'd never have sold out. I'd lay my own life on that. And believe me, Mr. Monk, when you work with a man on things like that"-he jabbed his thumb downwards towards the tunnels beneath them-"you get to know who to trust, and who not to. Get it wrong and you don't always live to talk about it."

"So both of the Argyll brothers must have known of Havilland's fears, and that he was possibly a danger?"

Sixsmith's face tightened and he pushed his hands into the pockets of his jacket. "I'm afraid so."

"And was Mary a danger also?"

Sixsmith considered for a moment before answering. "Not really. She had very little idea of what she was talking about.... Can't you call it an accident-Mary's death, I mean?"

Monk noticed that he had not mentioned Toby's death. "Both of them?" he asked. "Mary and Toby Argyll, too?"

A flash of understanding lit Sixsmith's eyes. "Would have to be, wouldn't it?"

"Well, if hers wasn't suicide, then his wasn't either," Monk said reasonably. "The only alternative would be murder. Could he have meant to push her over? She went over backwards, hanging on to him."

Sixsmith breathed out slowly. "Trying to save herself, or trying to pull him in with her, you mean?" His face brightened. "Changed her mind, and trying to save herself.' There you are. Unfortunately she was too late. Already lost her balance, and his too. Tragedy. Simple."

"You didn't say 'but Toby would never hurt her,' " Monk observed.

Sixsmith looked at him very steadily, and now his expression was unreadable. "Didn't I? No, I suppose I didn't. Got to get back to work now, Mr. Monk. Can't afford delays. Costs money. Good day." He walked away easily with a long, swinging stride.

Monk stood still for a moment, sharply aware again of the cold- and the noise of engines. The next thing he needed to ascertain was the exact time James Havilland had died, or as near as the police surgeon could tell him.