Forensic experts from the Kagawa Prefectural Police have developed a new method of obtaining fingerprints from human skin that they believe will be useful in identifying criminals in assault cases.

Although still in the experimental phase, the technology would be effective in narrowing down the suspects in cases such as those involving sexual assault or the abandonment of corpses, according to experts at the police force’s Identification Section and the Research Institute of Scientific Criminal Investigation.

Fingerprints are usually obtained by using a brush to sprinkle a fine aluminum powder onto a surface. The powder sticks to the oil left behind from finger secretions, and the shape of the prints appears.

Prints left on skin, however, are hard to identify because they are mixed with skin wrinkles, according to the experts.

To solve this problem, the police officers and researchers made use of the fact that the amount of oil left on the skin of a victim increases with the force of the attacker’s grip.

According to the experts, a rapist is likely to hold a victim down by grabbing hard on the wrists or neck. To collect fingerprints, investigators first press a plastic sheet onto the skin to take an impression of the oil and then spray a special chemical on it.

The oil reacts and turns blue. By heating it at a low temperature, the investigators can control the color contrast and achieve a clear image of the prints.

Makiko Miyazaki, 28, a police sergeant, took part in the experiments.

“I was very frustrated when girls my age were crying that the attacker grabbed their wrists. Even though I wanted to help them so badly, I was helpless because there was no way to collect fingerprints. That drove me to find a new technique,” said Miyazaki, the only woman on the four-member research team.

With advice from her colleagues, she experimented for a year by changing the order in which the substances were combined. She was finally able to obtain distinguishable prints.

“There are no new chemicals or devices. I just changed my point of view and switched the order, but it did take a long time,” she said.

The new technique will be put to practical use this year. “If a suspect is a former convict, we could nail him quickly by using the automatic fingerprint-recognition system,” Miyazaki said confidently.

But several problems remain. It is easier to collect prints from corpses because they no longer move or sweat. With living people, however, sweat can erase the fingerprints so the prints must be collected within four or five hours of the crime.

Moreover, sex attack victims usually wash away fingerprints by taking a shower after the attack to try to cleanse themselves mentally as well as physically. In addition, less oil is secreted in winter, and victims are likely to be wearing long-sleeved clothes.

The technique also needs to be used repeatedly in actual criminal investigations to make it more accurate and efficient, the experts said.

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