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.

Feng said that ultramicro DNA extraction is a method for extracting DNA from cast-off cells, such as skin residue or flakes, using a special kit when limited evidence is left at a crime scene. The kit was designed by Feng and a company in the city.

"The method makes our extraction procedure much simpler than traditional means, thus reducing the DNA's chance of being contaminated, which means we can get as pure DNA as possible, make multiple identical copies of the DNA sequence, and find the suspect," the 37-year-old Feng said.

Feng examined the scene again and extracted more than 30 cast off DNA samples. After careful comparison and analysis, he and his team confirmed that all the samples were from the same man.

Changchun police then collected more than 4,000 blood samples from the village and nearby areas for a comparative analysis and targeted a suspect named Zhou Xicai.

"How could you find me out? I have already cleared all the evidence!" Zhou reportedly told police while being arrested.

Zhou, a vegetable vendor, confessed that he killed the victims, a mother and daughter, two weeks after arguing with the mother.

Although less thrilling than what people see in the movie theater or on television, Feng and his team experience CSI: Crime Scene Investigation every day.

He dreamed of becoming a police officer when he was a boy, and became a forensic doctor after graduating from China Medical University in 2002. His career with the police rarely involves gunbattles or explosions, but patience and scrutiny instead.

An unsolved case in 2009 shocked Feng so much that he became addicted to DNA research.

An elderly couple were murdered in Dehui, a county-level city under Changchun. With the technology of the time, police could take only blood samples or cigarette butts to do a simple DNA investigation, but found nothing suspicious.

"If the case happened now, it could be solved by new technology. As years pass, trace elements have been decomposing, but I still cannot forget the appearance of the dead," Feng said with a sigh.

"There is only one truth," Feng said. "As police officers in technical investigation departments, we should make every effort to get evidence that is as solid as possible."