By the time the bodies of two women were found, all traces of evidence that could lead to their killer had been cleared.

This was the scene that the police saw this summer in a village near Changchun, the capital of Jilin province. Without any footprints, finger-prints or even cigarette butts, breaking the case seemed like a mission impossible.

But Feng Zhen, a forensic doctor with the Changchun police, simulated possible murder scenarios and extracted skin residue from the chaotic scene using a technique he invented called ultramicro DNA extraction. He eventually found an unknown man's DNA after testing.