The head of a Japanese prefectural police department in charge of investigating former death-row convict Hakamada Iwao says he plans to offer an apology on behalf of the police.

Hakamada, who is now 88 years old, was sentenced to death for the 1966 killings of a family of four in Shizuoka Prefecture, central Japan. He continued to claim his innocence after his death sentence was finalized in 1980. He was fully acquitted on Tuesday after prosecutors decided not to appeal a not-guilty ruling in his retrial last month.

The head of the Shizuoka prefectural police, Tsuda Takayoshi, told reporters on Wednesday that on behalf of the police in charge of investigations at the time, he is sorry that Hakamada was held for such a long time with an unstable legal status.

Referring to the bereaved families of the victims, Tsuda said it is regrettable that the true culprit was never identified.

The retrial ruled that three pieces of crucial evidence had been fabricated. Tsuda declined to comment on the ruling, saying that it is in line with Tuesday's statement by Prosecutor General Unemoto Naomi.

Tsuda said the police will learn from the case and work hard to regain public confidence by conducting appropriate investigations.