The fire began in the basement of the older north wing between about 2:00 p.m. and 2:20 p.m CST. Classes were due to be dismissed at 3:00 p.m. Ignition took place in a cardboard trash barrel located a few feet from the northeast stairwell. The fire smoldered undetected for approximately 20 minutes, gradually heating the stairwell and filling it with a light grey smoke that later would become thick and black, as other combustibles became involved. At the same time, it began sending superheated air and gases into an open pipe chase very near the source of the fire. The pipe chase made an uninterrupted conduit up to the cockloft above the second-floor classrooms (see "Evacuation" below).