Authorities in Japan have begun excavating the former site of a medical school that may contain the remains of victims of the country's wartime biological warfare programme. The school has links to Unit 731, a branch of the imperial Japanese army that conducted lethal experiments on prisoners as part of efforts to develop weapons of mass destruction. The Japanese government has previously acknowledged the unit's existence but refused to discuss its activities, despite testimony from former members and growing documentary evidence. Unit 731, based in Harbin in northern China, conducted experiments on tens of thousands of mostly Chinese and Korean prisoners, and a small number of Allied prisoners of war. Some historians estimate up to 250,000 people were subjected to experiments. According to historical accounts, male and female prisoners, named logs" by their torturers, were subjected to vivisection without anaesthesia after they had been deliberately infected with diseases such as typhus and cholera. Some had limbs amputated or organs removed. Leading members of the unit were secretly granted immunity from prosecution in return for giving US occupation forces access to years of biological warfare research. Some went on to occupy prestigious positions in the pharmaceutical industry, health ministry and academia.