The nation's highest prosecutor's office is poised to arrest two supervisors of a prosecutor accused of altering data seized in a postal abuse investigation, investigative sources said Tuesday.

In a rare criminal case involving prosecutors, facing arrest are Hiromichi Otsubo and Motoaki Saga, the former chief and deputy chief of the special investigation department of the Osaka District Public Prosecutor's Office.

They are suspected of hindering an investigation into prosecutor Tsunehiko Maeda, who has already been arrested.

Otsubo is currently the deputy chief prosecutor at the Kyoto District Public Prosecutor's Office while Saga heads an investigative department at the Kobe District Public Prosecutor's Office.