The contents of the report were so familiar to him that he put it down after no more than ten minutes. There was no point going over the details he had repeatedly studied.

Studying all the pictures once again, however, revealed a similarity between the masks of the first victim and third victim: both with a slight yellowish color. Some masks, if worn for too long, could have discolored, but the color for the two masks in question appeared to be deeper than that. It was possibly because of the material. With such severe air pollution, some high-end manufacturer could have pre-treated the material for better effect.

Sweating, he compared the four groups more closely.

For the second victim, the mask was definitely white. Also, he was wearing it when his body was discovered.

As for the fourth victim, Yu had just a couple of crime scene pictures from Qin in which the mask appeared to be partially covered by the earthen oven cake, with only its straps visible.

Under closer examination, however, the straps looked slightly yellowish too, though Detective Yu was not absolutely sure about the color.

In Qin’s case report, there was about half a sentence mentioning the first victim’s mask being a much-used one, off color. That made sense, considering her circumstances as a penny-saving night caregiver, but not so with the third. Nor with the fourth. If Xiang’s mask also proved to be yellowish, it would be too much of a coincidence – three out of four.

Rearranging the pictures on the desk, and restudying them in comparison, Yu was thunderstruck by another possibility for the first time: except for the second victim wearing the mask on his face, the masks for the other victims could have been planted by their bodies.

And that would be something too strikingly common among the victims. To use Chen’s word, the real ‘signature’, capable of telling them why the murderer had to commit the murder.

But how, Yu still had no clue.

After going over the pictures for another time, Detective Yu pulled out his phone and dialed Peiqin.

‘What’s up, Yu?’

‘Have you seen people wearing a sort of yellowish mask?’

‘Not that I can recall. But for young people a variety of patterns and colors would not be unimaginable. I have seen pinkish and florid ones, almost like colorful scarfs.’

‘Let me ask a different question. Is it possible for a mask to get yellowish after being repeatedly worn and washed?’

‘In my childhood, I used to wear and wash a mask so many times, so it’s usable for more than a year. It could have been a bit discolored – not as white as before – but never yellowish. Nowadays, masks are simply something disposable for young people. They don’t bother to wash them. But for your question, the answer is no, a mask won’t turn yellowish with wear and tear.’

‘So you are sure about it?’

‘I’m positive. But for all your questions about masks, I have a question for you too.’