A prison officer questioned in connection with a fatal assault on an inmate has told investigators that handcuffing the prisoner before assaulting him was "a justifiable part" of his duty, investigative sources said Tuesday.

A 49-year-old prisoner died in May and a 30-year-old inmate was seriously injured in September, reportedly following assaults by officers at Nagoya Prison. Earlier reports said both victims had been placed in restraining devices.

On Nov. 8, prosecutors arrested five officers at the prison, including Akihiko Maeda, 40, a deputy chief guard, on suspicion of inflicting serious injuries on the 30-year-old inmate.

They are also being questioned over the death of the 49-year-old inmate.

Maeda has told investigators that placing the 49-year-old prisoner in the restraining device -- leather handcuffs locked at the waist with a belt -- was justifiable.

A senior official at Nagoya Prison has told the prosecutors that he had advised the officers suspected of the assaults that their actions were legitimate, it was reported earlier this month.

The prosecutors plan to serve a second arrest warrant on Maeda on suspicion of fatally assaulting the 49-year-old prisoner.

One of the arrested officers has told investigators that it was inevitable that their actions would be censured as "problematic," the sources said.

Nagoya Prison has apparently used leather belts and manacles regularly to restrain prisoners who might try to escape, become violent or possibly attempt suicide, according to sources.