An inspection of construction sites is under way after a tower crane collapsed in Causeway Bay yesterday, killing two workers and injuring five. Six workers were trying to lower the height of the crane when it collapsed at the site of the former Mitsukoshi store in Hennessy Road. The crane buckled near the ninth-floor rooftop of the site, sending the operational box and horizontal arm crashing to the top of the building. It took firemen more than eight hours to free the body of one of the dead workers, while a survivor was trapped in the mangled wreckage for almost four hours. An investigation began once the rescue operation was completed, but already last night fingers were pointing at possible human error or faulty equipment. A crane expert said a failing hydraulic system was the likely cause. Leung Ping-fai, chairman of the Hong Kong Construction Professional Plant Operators and Mechanics Association, said the system appeared to have failed to maintain balance, leaving the crane tipping to one side and causing the column to snap. The Labour and Welfare Bureau said the main contractor - China Overseas Building Construction - must bear responsibility if the investigation showed it had failed to provide a safe environment for workers. Matthew Cheung Kin-chung, secretary for labour and welfare, said: 'Whether it involves errors in the construction work or human negligence, we will need a thorough investigation. 'I will direct the commissioner of labour to launch a city-wide inspection of all similar sites to see if safety protocol has been followed.' One worker, Lee Kong-chai, 42, suffered a broken leg when he fell from a platform in the basement when the building shook from the impact. Speaking at Ruttonjee Hospital, he said: 'I tried to hold onto a railing for support but then I heard a second loud noise. The sound was so great that I lost my grip on the railing.' It was the second fatal accident at the site. A worker was killed by an excavator in April. The two dead workers were identified as Tam Cheung-tai, 49, who was crushed between a partly demolished wall and part of the crane, and Tam Shing, 51, who was trapped within the crane's metal girders. Wong Tak-mon, 45, suffered leg injuries after tripping on the rooftop. He was discharged after treatment. Lai Yuen-ching, 48, was initially trapped in the collapsed crane but managed to climb out and was rescued by firemen at about 11am. He was in serious condition. Leung Ah-po, 55, was freed by firemen at about noon and was in serious condition. Chan Kin-wah, 36, the last survivor to be pulled clear at about 1.30pm, was in stable condition.