Prosecutors failed to properly investigate Iwao Hakamata, 88, who was acquitted this year in his retrial for the murder of four people in 1966, the Supreme Public Prosecutor's Office said in a report on Thursday.

"We are very sorry that Mr. Hakamata and his older sister, Hideko, spent days so painful they can't be expressed in words," Deputy Prosecutor-General Hiroshi Yamamoto said at a news conference on the day.

The report points out that police interrogations were conducted without Hakamata's willingness as some were carried out in the middle of the night and others were prolonged and threatening. It notes that prosecutors should have secured such willingness by taking sufficient measures in their own interrogations.