A detailed map has been found in the U.S. of underground bunkers dug by the Imperial Japanese military on Iwojima, a discovery that could aid the retrieval of remains of more fallen soldiers on the major World War II battleground, government officials said Friday.

About 21,900 Japanese soldiers are believed to have died on the remote Pacific island now known as Iwoto and virtually uninhabited. But the remains of only about 9,900 have been recovered, partly because an airstrip and other Self-Defense Forces facilities have been built there.

The detailed map was found in early April by government-appointed researchers as they sifted through documents from the U.S. National Archives. The map, apparently drawn by the U.S. military, shows 95 entry points to bunkers.

The underground bunkers are believed to stretch 18 km if linked together, the sources said.

With the map showing several bunkers running underneath the SDF runway, the Defense Ministry is planning to study the area with sonic gear, given survivors’ testimony that many remains are still unearthed, the sources said.

The government plans to complete an intensive search for remains on the island by the end of March 2014.

The bunkers symbolize the Japanese military’s costly drawn-out battle during the war. The U.S. side attacked the bunkers with flame throwers and blew up or buried them as soon as their entrances were found, the sources said.

The government-appointed team of researchers suspects the underground bunkers hold the remains of a significant number of Japanese soldiers, given the likelihood that no other mass graves exist elsewhere on the island than the two sites already found, the sources added.