On Sept. 10 the Osaka District Court acquitted Ms. Atsuko Muraki, a former welfare ministry’s bureau chief, of instructing her subordinate, Mr. Tsutomu Kamimura, to fabricate and issue a certificate that recognizes an organization as a group for the disabled, thus enabling it to use a postage discount system. The ruling cast doubt on the legitimacy of the investigation. The arrest Tuesday of the elite investigation team’s chief prosecutor, Mr. Tsunehiko Maeda, on suspicion of having altered the content of a floppy disk seized from Mr. Kamimura further undermines people’s trust in public prosecutors. (The prosecution decided not to appeal the ruling that declared Ms. Muraki innocent.)

In hearings, Ms. Muraki’s codefendants and witnesses withdrew statements they had made during the investigation and the court refused to admit as evidence 34 of 43 depositions presented by the Osaka District Public Prosecutors Office’s special investigation squad, pointing to the possible use of leading questions or pressure.

The floppy disk, seized on May 26, 2009, contained text of the fabricated certificate. The court adopted as evidence an investigative report that Ms. Muraki’s defense counsel obtained from the prosecution. It said that the disk was last updated at “1:20.06” a.m. on June 1, 2004. The prosecution argued that Ms. Muraki instructed Mr. Kamimura, also on trial, to issue the certificate during the first 10 days of June. The defense counsel pointed out that the disk update of shortly past midnight May 31 contradicted the argument.

After the floppy disk was returned, it was found that the disk had been falsified, with the last update changed to “9:10.56” p.m. of June 8. If the disk had been submitted to the court as evidence, Ms. Muraki could have been found guilty. The possibility cannot be ruled out that Mr. Maeda tampered with the disk to make its content congruous with the prosecution’s preconceived scenario.

The Supreme Public Prosecutors Office, investigating the disk falsification, should not trivialize it as an error of an individual. Violation by prosecutors of the principles of investigation could destroy the basis of criminal trials.

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