The West Midlands Serious Crime Squad, which was responsible for the Birmingham Six investigations, attracted further controversy after other convictions were questioned, and was closed down in 1989. Around 40 prosecutions collapsed due to malpractice in the 1980s, and a further 60 convictions have since been quashed, mostly because of tampering with suspects' statements, to add in 'confessions'. Many cases also depended on 'Supergrass' evidence which has since been found to be highly unreliable. Some of those wrongfully convicted alleged torture, including use of a suffocation technique known as "plastic bagging".