The bombing proceeded in three waves from 21:25 to 21:42. First the roofs and windows of the old town were destroyed with 256 heavy bombs and aerial mines (396 tons). This prepared the way for 300,000 incendiary bombs (582 tons). Within a very short time isolated pockets of fire emerged, and this developed into a single area-wide firestorm with temperatures from 1500-2000 °C. The population had only minimal warning, and fortified bunkers were seldom available. For most, the best option was only a provisionally prepared basement shelter. To facilitate locating these shelters, buildings were marked on their walls SR/LSR for shelter, NA for emergency exit, and KSR for no shelter. These markers may still be found today scattered among the buildings. To avoid being caught in the fire-storm or suffocating, many people ran for their lives and tried to reach the banks of the Main river or the edge of town. The fire department was faced with a hopeless battle and tried to contain it with water lanes (Wassergasse). On the approach to Würzburg one Lancaster was shot down by a German Night fighter and five more RAF Lancasters were lost during or after the attack. In the ruins of the city in the days which followed 3000 dead were recovered or identified, and an additional 2000 unregistered refugees are believed to have perished under the rubble.