TOKYO, June 9 (Xinhua) -- Information on patients and staff at two hospitals in Japan's northernmost prefecture of Hokkaido was leaked after a local waste disposal company improperly handled discarded hard drives, potentially affecting as many as 510,000 people, local media reported Monday.

The issue came to light after a purchaser of a hard drive at an online auction site informed authorities last June that the device contained data belonging to the Hokkaido Medical Center.

The hospital subsequently recovered similar hard drives and launched an investigation, according to public broadcaster NHK.

The investigation found that 31 of the hard drives had been used by the Hokkaido Medical Center and two by the Hokkaido Cancer Center to store data, including electronic medical records.

The hard drives were confirmed to contain personal information on about 187,000 patients and staff, including names, addresses, and medical diagnoses, while up to 510,000 people may be at risk of having their data compromised, the report said.

The two hospitals had previously contracted the same waste disposal company in the city of Ishikari to destroy the storage devices. However, the company failed to do so, allowing the hard drives to enter the second-hand market.

Japan's National Hospital Organization, which manages the two hospitals, has reported the case to the police and filed a criminal complaint against the waste disposal company.