According to local media, Vo Nhat Thang, chairman and general director of Long Hoi Electricity Construction and Investment JSC said the wooden ceiling molds used during the construction of the tunnel had decayed, causing rocks and concrete to fall.

"Ongoing rains for more than a month also contributed to the collapse," Thang said.

And also blamed the area’s poor geological integrity for the incident, which happened early on 16 December during a heavy storm. Twelve workers were trapped inside a tunnel as the water rose.

They all were freed on 19 December by a team of 750 rescue workers.

"The geological formation in this area is very complicated. Two experienced contractors – Lung Lo and Vinaconex – began work on the tunnel, then quit for that reason," said Thang.

Minister of Construction Trinh Dinh Dung has sought government permission to investigate the cause of the tunnel collapse. "This accident was complicated. It was linked to problems with the surveying, design, construction, and supervision of the project. For these reasons, we want to lead the probe instead of the provincial authorities," said Dung.

The project’s design consultant is China’s Nanning Hydropower Irrigation Design Institute and the supervisor is Hanoi-based Nhat Thang Consulting JSC.