The Khmer Rouge converted a disused secondary school in Phnom Penh's Tuol Sleng region into a security prison, S-21. It was placed under the responsibility of the defence minister, Son Sen. The numbers sent to S-21 grew steadily as the CPK purge proceeded. In the first half of 1976, about 400 people were sent there; in the second half of the year that number was nearer to 1000. By the spring of 1977, 1000 people were being sent there each month. Between 15,000 and 20,000 people would be killed at S-21 during the Khmer Rouge period. About a dozen of them were Westerners. Pol Pot never personally visited S-21.