Members of the House of Councilors Judicial Affairs Committee visited Tokyo's Fuchu Prison on Thursday to look into its handling of official inmate records.
Authorities at the prison, on the outskirts of the capital, claim to have lost documents on nine inmates who died there between 1996 to 2000, prompting speculation the records may have been deliberately removed.
Fuchu Prison accounts for 93 of the 1,592 deaths in the last 10 years at prisons and detention centers nationwide. The figures were made available following the arrest of guards at Nagoya Prison on suspicion of fatally abusing inmates.
The Upper House committee questioned prison officials about how they kept the documents, which include details such as whether restraining devices were used.
The Justice Ministry suspects the missing documents were deliberately removed and may file a criminal complaint if ministry investigations support its suspicions, according to officials.
On Wednesday, the House of Representatives Judicial Affairs Committee inspected Nagoya Prison, where several fatal assaults on inmates by guards have been revealed. The committee was critical of the prison's medical system.