“Perhaps. It was locked when we got here yesterday. But it could have been locked by one of the other residents if they found it open in the morning.”

They craned to see round the wall of the lift shaft, but it effectively blocked off their view of the lobby and the main door. “So Chao wouldn’t have seen his visitor until he was right at the gate,” Margaret said. “Wouldn’t have seen him crossing the lobby and got alarmed when it wasn’t who he was expecting.”

“Hold on. I think I might have made a mistake there,” Li said suddenly. “I assumed that Chao was expecting someone else and that his killer was unknown to him. But if it was someone he already knew, a new supplier maybe, or someone he believed might provide him with young boys, there would be no reason for the killer to tip his hand at that stage.”

“And if Chao knew him he would probably have invited him up,” Margaret said.

“So he didn’t have to be forced up the stairs at gunpoint.” Li began to think there might be virtue in this exercise after all. How often had his uncle told him that the answer almost always lay in the detail.

They went back up the stairs and into the apartment, stopping by the bloodstain on the hall carpet. “The killer wasn’t going to hang around making polite conversation,” Margaret said. “It looks like he hit him on the head as soon as they got in. The size of the contusion and fracture would be consistent with your idea that he might have used the barrel of his gun. And he would have injected him with the ketamine straight away. He couldn’t know just how hard he had hit Chao, or how long he would be unconscious. He would have pulled off his left shoe, stripped back the sock and followed Chao’s well-worn needle path into the bloodstream. So either he knew him well, or had been very thorough in his research. He pulled the sock back up and replaced the shoe. Then what?”

“He waited,” Li said.

“Why?”

“He would have some time to kill before dawn. Safer to wait here than in the park.”

“Okay. But he needed to be in the park with Chao by sun-up.”

“Is it not true that the darkest hour is just before the dawn?” Li asked.

“I guess it is,” Margaret conceded. “And I’ve had plenty of opportunity to put it to the test the last couple of nights.” She thought for a moment. “So he left, with Chao, some time between three and four a.m. In time to get him into the park under cover of darkness and be there when it opened. How did he get him down the stairs?”

“Probably over his shoulder.”