The Supreme Public Prosecutor's Office on Thursday questioned the supervisors of an Osaka prosecutor under arrest on suspicion of falsifying investigative documents to determine if they were aware of the crime, investigative sources said.

The supervisors of Tsunehiko Maeda, 43, include Hiromichi Otsubo, former chief of the special investigation department at the Osaka District Public Prosecutor's Office. The office investigated a postal abuse case involving a senior welfare ministry official who was recently acquitted.

Although the supervisors were informed of allegations that Maeda tampered with data seized in connection with the probe, it is believed they failed to investigate or disclose those allegations.

"I myself was not involved in the falsification. It is my supervisory responsibility that will be at issue," Otsubo, now deputy chief prosecutor at the Kyoto District Public Prosecutor's Office, said before questioning.

Maeda was arrested Tuesday on suspicion of altering computer data related to the postal probe.

Meanwhile, Atsuko Muraki, a senior official at the Health, Labor and Welfare Ministry, was reinstated following finalization of her acquittal. The government plans to appoint her director general for policies on cohesive society at the Cabinet Office.