The impact of rescue efforts on much of the earthquake disaster zone in Sichuan had made it all but impossible to find out why so many buildings collapsed, an official said yesterday. Tang Kai , head of city planning at the Ministry of Housing and Urban-Rural Development, was speaking at a press conference to discuss the rebuilding efforts. Determining why so many structures collapsed during the magnitude-8 quake has been the most controversial issue surrounding the May 12 disaster. Many of the dead were children, who were killed when an estimated 7,000 schools toppled. Until this week, the government had resisted saying how many pupils died. But on Thursday the provincial government announced 5,335 students and kindergarten pupils were either killed or missing, accounting for 6 per cent of the total 87,449 quake victims. This figure was lower than what many people had estimated. Parents blame shoddy construction and official corruption, and have tried to pressure the government to hold someone accountable. But their pleas have been ignored and the courts have yet to accept lawsuits from parents and activists. A senior official in Sichuan suggested on Wednesday that no officials should be held to account for problems with building quality. Provincial Justice Department director Liu Zuoming said the intensity of the quake was beyond anyone's control and the large-scale collapse of schools was the result of flawed building standards. Mr Tang said: 'It is regrettable the mechanism of earthquakes is different when it happens at different places. It is difficult to tell its impact on buildings.' He said many partially destroyed buildings were pulled down in the rescue effort and the government could only rely on building plans to examine why they had collapsed. This led to difficulties in pinpointing whether the problem was caused by design, construction, poor monitoring, or inappropriate use. Two weeks after the quake, the mainland's product quality watchdog said it was collecting samples of construction materials from collapsed schools for investigation. Zhi Shuping, vice-minister of the State Administration of Quality Supervision, Inspection and Quarantine, said it would investigate the quality of materials used in collapsed schools. The government later said the inquiry did not yield any useful insights. The quake left 5 million people homeless. Much of the reconstruction has been aimed at rebuilding homes, schools and hospitals. Nearly 250 officials in quake-affected areas were found to have misappropriated relief funds, according to Xinhua. Mu Hong , vice-chairman of the National Development and Reform Commission, said that by the end of last month, about 360 billion yuan (HK$409 billion) had been invested in more than 21,000 projects in Sichuan, Gansu and Shaanxi . Mr Mu said government assistance had been distributed evenly in the disaster areas despite some places gaining more public attention.