As investigators tried to determine the cause of a bus crash that killed 36 people when the vehicle struck the mouth of an expressway tunnel in Northwest China's Shaanxi province on Thursday, an engineering expert has questioned whether the tunnel design might have posed a hazard.

The accident occurred at about 11:30 pm on Thursday in Ankang when the bus hit the wall at the mouth of No 1 Qinling Tunnel on the Xi'an-Hanzhong section of the Beijing-Kunming Expressway, according to the State Administration of Work Safety. In addition to the 36 deaths, 13 people were injured, authorities said on Friday.

"Generally speaking, when entering a tunnel, the width of the lanes approaching the tunnel should be the same as the width inside the tunnel," an engineer in Chengdu surnamed Lei told the news portal thecover.cn.