At least 3,000 civilians died in 1945 and 1946 in camps set up by the U.S. military in Okinawa, according to a survey shedding light on their little-known plight in the facilities.

It was known that many Okinawans died in these camps during and after the war, but the Okinawa Prefectural Government says the exact number is a mystery due to a lack of official documents.

The survey — in which questionnaires were sent to all 41 municipal offices in the prefecture — found that the main causes of death were starvation and malaria, as the camps were in unsanitary surroundings and little food was provided. The survey results were released Saturday.