Four residents of Choshi, Chiba Prefecture, died in the early 1950s from poison gas believed produced by the Japanese military before the end of World War II, according to a city document released Thursday.

The city is located about 23 km southeast of Kamisu, an Ibaraki Prefecture town where 20 residents currently suffer from illnesses suspected of being linked to poison gas abandoned by the Imperial Japanese Army after the war.

The Choshi municipal government came across the documents while re-examining records on past cases, which was done because the city is located close to Kamisu, city officials said.

The Environment Ministry knew nothing of the deaths because few documents remain from the time, a ministry official said. But the ministry plans to ask municipalities nationwide to follow Choshi's example and review their records.