A recent court acquittal of a 23-year-old Kagoshima man who had been charged with rape has again showcased the questionable handling of evidence — DNA analysis this time — by both the police and prosecutors. If the court had overlooked the suspicious behavior of investigative authorities, the man might have had to serve four years in prison due to a false charge.

Both the police and prosecutors need to review their handling of the evidence in question and make public detailed reports so that the case can serve as a lesson to ensure fairness in investigations. The outcome of the case underlines the potential risks of leaving control of physical materials for DNA tests in the hands of investigation authorities. The government should devise a system in which such materials will be kept and DNA-tested in a transparent manner.

The man was indicted on a charge of raping a 17-year-old girl in an entertainment district of the city of Kagoshima in 2012. The crime laboratory of the local police said it attempted to perform a DNA test of the semen found inside the woman’s vagina but was unable to carry out an analysis because the size of the sample was too small. However, the man’s DNA was detected in saliva found on the woman’s breast. He said that he had no memory of what he did because he was drunk.

In 2014, the Kagoshima District Court found the man guilty, on the grounds that the saliva finding supported the woman’s claim that he raped her even though the existence of his DNA in the saliva did not necessarily mean that the semen was his.

A turning point of the case came in the appellate trial at the Miyazaki branch of the Fukuoka High Court, in which the man’s defense called for a new DNA test and the branch court asked an expert to conduct a fresh DNA test on the semen. The test showed that the semen was not the defendant’s and this new evidence eventually led to the man’s acquittal.

In its ruling, the presiding judge at the high court branch censured the staff of the police crime lab for having disposed of the DNA solution that it used for the test and the memo that recorded the DNA test process. Since the DNA solution was key evidence, the lab staff’s action is shocking and deplorable.

The judge went on to say that either the testing skills of the police lab staff were poor or it had detected that the DNA belonged to a different person but decided instead to support the investigator’s case by reporting that the sample was too small to analyze. Either way, doubt has been cast on the credibility of DNA tests by the Kagoshima police. The judge’s statement points to the possibility that the Kagoshima police attempted to hide key evidence. If true, this is despicable behavior that undermines the fairness of criminal trials.

A dubious act by the prosecution also surfaced in the course of the appellate trial. After learning of the results of the new DNA test, the prosecutors had yet another expert carry out an analysis using the same material — without informing the court or the defense counsel. The judge lashed out at the prosecutors for their action and secrecy, and pointed out that it was reasonable to suspect that the prosecution intended to disclose the results only if the outcome turned out to be favorable to their position — a course of action that would undermine the trial’s fairness. He went on to say that the prosecution might have tried to use up the scarce material by conducting a test that was “unnecessary” and “not urgent.”

After the prosecution decided not to appeal and the high court ruling was finalized, the National Police Agency issued an instruction to police organizations nationwide, requiring them to perform a DNA test even if the quantity of a sample is small and to keep a detailed record on the test process. But the NPA should realize that what happened in the Kagoshima case raises doubts on whether the police can be trusted to carry out DNA tests in an accurate and transparent manner. The government should consider establishing a system in which biological evidence in criminal cases will be kept in a secure manner to prevent tampering and DNA tests will be carried out transparently, including in the presence of independent experts and at the request of defense lawyers.

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