Doctors were last night battling to save a man who is believed to have turned his flat into a gas chamber, killing his wife, three children and two male tenants. According to police, Chan Ying-cheung, 41, drugged the men, his wife Ng Suk-yee, 29, his daughter Chan On-yee, six, and son Chin-bong, three, before gassing them. Also among the dead was his 21-year-old son from a previous marriage. Police said the former plumber drilled holes in two doors so he could feed gas into the bedrooms through a tube connected to the flat's central LPG supply. He was in intensive care at Yan Chai Hospital last night in critical condition. The dead men were migrants from Fujian - Wong Wing-fat, 32, and Cheung Shun-leung 40, - who had sub-let a room from Chan. Police were called to the fourth-floor flat in Tai Yuan House, Tsuen Wan Centre, at 8.40 am after residents reported smelling gas and said they could see bodies inside. When firemen broke into the 492-square-foot flat they found the three men in one bedroom. Chan was unconscious in an adjacent bedroom with the bodies of his wife and two younger children. The stem of a four-metre Y-shaped plastic tube was connected to the gas main in the kitchen with the other ends pushed through the holes in the bedroom doors. Police found a bottle believed to have contained sleeping pills in Chan's bedroom. They also took away several water bottles in which it is believed the drugs were dissolved. Several letters were also found scattered on the floor. Chan, who had his own plumbing business, rented the flat two years ago for $4,500 a month. Neighbours said he had sub-let one of the rooms after his business failed. His landlady said Chan had not paid the rent for three months and had been told to move out. He had not paid the maintenance fees since March. 'His telephone line had been disconnected and the doorbell was also not working, possibly to avoid debtors,' she said. The family arrived in Hong Kong from Fujian just over two years ago. Neighbours said they had recently heard the couple arguing. A security guard, Lee Lap-pui, said he had received one complaint a few months ago about noise from the Chan flat after midnight. 'They were quite co-operative. They stopped making the noise after one warning. They said their children were playing,' he said. Neighbours said the Chans generally kept to themselves, seldom speaking to other people except for an occasional morning greeting. One neighbour said: 'I occasionally came across the woman in the lift. She was quite nice but recently she and her husband had been quarrelling.' The police have provisionally classified the case as attempted suicide and murder. Postmortem examinations will be performed today. Senior Superintendent Ian Mackness said: 'At this stage, no evidence proves any outside persons were involved.'