When a building in a densely developed urban area falls down and kills people, the community rightly expects experts with powers of investigation to determine how it happened. Only in this way can the lessons of the tragedy be applied so that it is not repeated. The report into the collapse of a block of flats in To Kwa Wan in January falls short of this, leaving key questions unanswered. Transparency is crucial; if a reason can't be given, we need to know why. The Buildings Department's preliminary report is only six pages long. And its findings are vague when it comes to the central question of what caused the collapse. Director of Buildings Au Choi-kai told lawmakers it was due to 'external forces'. But further tests are needed to establish what those external forces were. Au unhelpfully said elaboration was not possible because of legal advice that the department had received. Perhaps he is worried about prejudicing a future criminal trial. If so, that was not made clear. There was no confirmation that workers who had been demolishing structures on the ground floor were employed by a registered contractor. All we know after the months of investigation is that three steel structural columns seem likely to have failed for an undetermined reason, bringing the five-storey tenement crashing down. Relatives of the four people who died are, understandably, not satisfied. They have lost loved ones and breadwinners, property and their homes. Legal action that leads to compensation is uppermost in the minds of some; they, no doubt, worry this won't be possible without human error having been determined. The owner and workmen who visited the building also don't know where they stand. But understanding what happened at 45J Ma Tau Wai Road is not only a matter for people with a direct relationship to the building - we all need to know. Inspectors had given it a clean bill of health, just as has happened with thousands of other structures in our city over the past year. Until the precise cause is revealed, doubts will continue to be raised about the safety of our buildings. Authorities have admitted there are problems in carrying out inspections. The Buildings Ordinance sets out strict rules on upkeep and renovation of buildings and flats, but enforcing them is at times patchy. Access to some premises is sometimes not possible. Ensuring that only registered builders are involved in work is logistically challenging. In the wake of the collapse, 40 inspection teams were sent out to urgently check the 4,000 buildings in our city that are 50 years or older. Investigations of debris from the tenement continue. Officials say a report on the forensic findings will be available in a month. The focus is on a structural column at the rear of the ground floor that was found to have been damaged. This appears to have led to extra load being put on the remaining supports; the preliminary study suggests that this was too much for them to bear, causing the collapse. The tragedy came as a shock to Hong Kong - in a high-rise city, we expect building regulations to be as good as any in the world and enforcement to be equally sturdy. That three months later we are no wiser as to how or why they failed in To Kwa Wan is disappointing. We don't expect the investigation to be rushed - rather, we want it to be done properly. Perhaps there won't be answers to every aspect, but at the least we expect complete disclosure. To prevent another such tragedy, we need to know and understand.