The probe by the ministry's Inspector General's Office of Legal Compliance has found that Inada did not receive a written report on the existence of the reports in electronic form within the GSDF or was not asked to approve the nondisclosure policy for the documents when she received briefings on the matter from senior officials in mid-February.

At the same time, the office's investigation report, released on Friday, said the possibility could not be ruled out that during the briefings by ministry and GSDF executives, Inada heard about the existence of the documents, which the GSDF had denied.

An early draft of the investigation report did not touch at all on the issue of whether Inada was involved in the cover-up, but the GSDF side reportedly claimed during the investigation that the then minister knew that the data were kept in the GSDF, according to informed sources.

Inada and other politically appointed ministry leaders were not subject to the special investigation, which was ordered by Inada herself. But she accepted a hearing by investigators following media reports alleging her approval for keeping the existence of the data undisclosed.

Still, the inspector general's office failed to clear up the suspicion about her involvement. The office may have faced limitations in the probe, because it does not have the authority to conduct compulsory investigations.

On Friday, Inada resigned as defense minister to take responsibility as the supervisor of the ministry and the GSDF.

Opposition parties criticized the government's response to the cover-up scandal.

"It's extremely problematic that Inada's resignation may end up leaving everything shrouded in darkness," Hiroshi Ogushi, policy chief of the Democratic Party, stressed.

With the opposition camp demanding an off-session parliamentary meeting on the scandal, Ogushi said, "Her attendance as a witness is indispensable."

"Inada has a duty to fully answer questions in the Diet," Akira Koike, chief of the Japanese Communist Party's secretariat, told a press conference.

Both Ogushi and Koike said Prime Minister Shinzo Abe should be held responsible as the person who appointed Inada as defense minister. (Jiji Press)