OITA (Kyodo) Three workers who died Saturday after collapsing inside an ore carrier vessel docked at a smelter in Oita, southwestern Japan, suffocated due to lack of oxygen, police said Sunday.

The Oita prefectural police made the announcement after inspectors of the Land, Infrastructure, Transport and Tourism Ministry's Japan Transport Safety Board began an on-site examination of the ship at the facility of Nikko Seiren, a Tokyo-based smelter of nonferrous metals.

The three, all employees of a Nikko Seiren subsidiary, lost consciousness and collapsed shortly before 9 a.m. Saturday as they entered the ship's hold that contained about 5,900 tons of copper concentrate, and were confirmed dead at a hospital, the police earlier said.