Director of Buildings Au Choi-kai received a grilling yesterday from legislators dissatisfied with the lack of a concrete finding in Monday's preliminary report of the investigation into January's fatal collapse of a To Kwa Wan tenement building. Au parried repeated questioning on the inconclusive report - which found 'external forces' responsible for the collapse - by saying the Buildings Department had received legal advice that disclosure of further details at this point could hinder future legal action. He told the Legislative Council that forensic tests were still to be completed on three steel support columns that failed in the collapse, to determine whether they had been cut by somebody or were just torn apart naturally. The results would be available in a month. Legislators pressed Au to reveal more about the nature of the 'external forces'. 'The external forces can't be the power of God or a ghost. They must come from a person,' Lee Wing-tat of the Democratic Party said. 'The victims' families have waited for three months. The government should clarify the details or they will be treated unfairly.' Raymond Ho Chung-tai, who represents the engineering sector, said the investigation was taking an unreasonably long time. 'Forensic engineering just involves simple tasks,' he said. 'You just need to look at the remnants [of the columns] and see if they were cut off naturally. It doesn't need three months of tests.' The chairman of the subcommittee on building safety, Patrick Lau Sau-shing, urged the government to make public more details. The report says structural analysis ruled out subdivision of flats or poor maintenance as causes of the collapse of 45J Ma Tau Wai Road, which cost four lives. But apart from its vague reference to 'external forces', it says little else about the causes. Lawmakers asked Au what kind of work was being carried out by workers on the ground floor on the day of the collapse, whether the workers belonged to a registered contractor, what the origin of the external forces was, and whether the case involved a criminal element. Au said police were carrying out a criminal investigation but he could not give other details: 'We sought legal advice to see which information can be disclosed,' he said, adding that improper disclosure could affect future legal action. He said the department had no records showing the contractor hired by the building's owner was registered. But he could not disclose whether new information had arisen from police questioning of witnesses. Secretary for Development Carrie Lam Cheng Yuet-ngor said Au's cautious attitude was appropriate. 'The investigation has yet to be completed,' she said.