the volume of material destroyed in the final days of the regime, the office of the Federal Commissioner for the Stasi Records (BStU) believes that there could have been as many as 500,000 informers. A former Stasi colonel who served in the counterintelligence directorate estimated that the figure could be as high as 2 million if occasional informants were included. There is significant debate about how many IMs were actually employed. Infiltration Full-time officers were posted to all major industrial plants (the extent of any surveillance largely depended on how valuable a product was to the economy) and one tenant in every apartment building was designated as a watchdog reporting to an area representative of the Volkspolizei (Vopo). Spies reported every relative or friend who stayed the night at another's apartment. Tiny holes were drilled in apartment and hotel room walls through which Stasi agents filmed citizens with special video cameras. Schools, universities, and hospitals were extensively infiltrated, as were organizations, such as computer clubs where teenagers exchanged Western video games. The Stasi had formal categorizations of each type of informant, and had official guidelines on how to extract information from, and control, those with whom they came into contact. The roles of informants ranged from those already in some way involved in state security (such as the police and the armed services) to those in the dissident movements (such as in the arts and the Protestant Church). Information gathered about the latter groups was frequently used to divide or discredit members. Informants were made to feel important, given material or social incentives, and were imbued with a sense of adventure, and only around 7.7%, according to official figures, were coerced into cooperating. A significant proportion of those informing were members of the SED. Use of some form of blackmail was not uncommon. A large number of Stasi informants were tram conductors, janitors, doctors, nurses and teachers. Mielke believed that the best informants were those whose jobs entailed frequent contact with the public. The Stasi's ranks swelled considerably after Eastern Bloc countries signed the 1975 Helsinki accords, which GDR leader Erich Honecker viewed as a grave threat to his regime because they contained language binding signatories to respect "human and basic rights, including freedom of thought, conscience, religion, and conviction". The number of IMs peaked at around 180,000 in that year, having slowly risen from 20,000 to 30,000 in the