4. Filters investigated

On November 4, 1986, 112 used filters were collected from various containers both at and near the site of the fire. It was not possible to match single filters with their original users. However, the locations of the containers gave indirect evidence which fire brigade or which workers of the clearing squad had used the filters, and whether the filters were used on the day of the fire or afterwards. Filters found in the same container were designated as a batch. All filters were stored in air-tight plastic bags, originally at room temperature, after November 1986 at ca. -20°C under N2.