An old woman was today killed and dozen others including children were injured when the upper floor of a single-storeyed building came crashing down to the ground due to a gas cylinder explosion, disaster management officials said. The deceased was identified as Tarabai Gaikwad and all the occupants in the building were injured. The injured comprising two men, four women and six children have been admitted to local as well as Sion hospital in Mumbai, they said. According to Regional Disaster Management Cell (RDMC) chief Santosh Kadam, the incident took place around 8 AM in Adharwadi locality of Kalyan after occupants on the first floor switched on an electric switch and the cylinder exploded causing fire. The cylinder was apparently leaking throughout the night, he said. The residential structure, which collapsed completely after the fire and explosion, was housing three families. A team of disaster personnel and firemen besides policemen have rushed to the spot and rescue operations are underway. Further details are awaited.