Three women died and 21 other people - 17 of them fire officers and ambulancemen - were injured yesterday when a fireball tore through a flat where a woman was apparently trying to commit suicide. The explosion, which rocked the 22-storey building in Tsui Chuk Garden, Wong Tai Sin, and triggered a ferocious blaze, took place seconds after firemen alerted to the suicide attempt broke down the door of the flat in a rescue operation. Two female neighbours on the 15th floor, Ngan Hau-lan, 75, who lived in block D, and Sze Tsui-ngor, aged about 60, who lived in block C, were declared dead. In the evening, the charred body of another woman was found inside the kitchen of the flat where the blast occurred in block E. Last night officers were trying to identify her. All three flats are close to each other. Police said they are looking for the male occupant of the flat in block E, Chan Fuk-sang, aged between 72 and 80. The woman found in the kitchen was believed to be his wife. Four firemen were seriously injured with burns to their faces while another seven were in stable condition. Five other officers were treated and discharged while another was under observation in hospital. Four other residents in the block of flats were injured. Several of the rescue workers who were injured were in a lift on the way to the scene when the explosion sent it plunging from the fifth floor to the second. An investigation was launched by the Fire Services Department into the handling of the incident. Last night, Secretary for Security Regina Ip Lau Suk-yee expressed serious concern over the incident and pledged a thorough investigation, including the rescue procedures adopted. She later visited the injured firemen in hospital. Glass and debris showered down from the apartment block shortly after 3pm when the explosion took place. Residents said they first thought it was thunder. They felt the building shake as the explosion tore through it. Emergency crews were called to the flat in block E when the 43-year-old son of an elderly couple living there could not get into the apartment and smelled liquefied petroleum gas (LPG) spreading from it. The building is equipped with a centralised LPG system. Police confirmed last night that the front door of the 15th-floor flat was locked at the time of the fire. 'Our officers broke into the unit. As five firemen were entering it, the explosion suddenly occurred,' said deputy chief fire officer (Kowloon) Chan Yun-pui. Mr Chan said damage was extensive and the eight other flats on the floor were also affected. A fleet of 27 fire engines and 11 ambulances were sent to the scene and about 160 firemen were deployed. The housing estate was shrouded in dense smoke. More than 100 residents were evacuated from the building and firemen used three jets to spray water into the burning unit. The blaze was put out after about 1.5 hours. A housewife who heard the explosion and ran out of her second-floor unit, said she found the lift doors open and several firemen and ambulancemen with blackened faces crawling out. They ordered her to run from the building. Mr Chan said preliminary information indicated that firemen had taken all the required safety measures in breaking into the unit. 'In order to prevent any sparks, our officers were spraying water on the door as they broke into the unit,' he said. Raymond Ho Chung-tai, an independent legislator who represents the engineering profession, said there should be an official investigation into the cause of the fire.