The lead prosecutor in the postal-abuse indictment of senior welfare official Atsuko Muraki was arrested Tuesday after he admitted to tampering with data related to the case.
Tsunehiko Maeda, 43, was arrested in a swift move by the Supreme Public Prosecutor's Office, which launched an investigation earlier in the day.
He is suspected of altering the date that information on a floppy disk was updated, sources said. The disk was seized in May 2009 from a Muraki subordinate when he was questioned by other prosecutors, they said.