The Supreme Public Prosecutor's Office on Friday arrested two supervisors of a prosecutor who was taken into custody last month for allegedly tampering with evidence, investigative sources said Friday.

Arrested were Hiromichi Otsubo, 57, and Motoaki Saga, 49. The two used to be chief and deputy chief, respectively, of the special investigation department at the Osaka District Public Prosecutor's Office.

Otsubo and Saga are suspected of hindering the prosecution of Tsunehiko Maeda, 43, who was arrested on Sept. 21 in connection with his probe into a postal abuse scandal. Both denied the allegations, prosecution sources said.

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

Meanwhile, investigators are considering questioning Takumi Nakao, the former head of the Osaka High Public Prosecutor's Office, as a witness in the data-tampering scandal, the sources said. Nakao retired in June as head of the office, which has control over the Osaka District Public Prosecutor's Office.

Earlier Friday, investigators from the Supreme Public Prosecutor's Office continued their second grilling of Otsubo and Saga, which began Tuesday.

The Osaka District Court also allowed the investigators to detain Maeda for another 10 days though Oct. 10.

Maeda, who led the investigation into the postal abuse case, was arrested Sept. 21 on suspicion of destroying evidence after Atsuko Muraki, a senior welfare ministry official, was acquitted on Sept. 10.