Police are investigating further.
The Thu Thiem Tunnel is an underground road tunnel designed to cross the Saigon River in Ho Chi Minh City, the largest city in Vietnam.
The tunnel and associated highway were constructed to link the Thu Thiem Peninsula to National Highway 1A and provide a new integrated transport route for the country.
The project began in 2004 with $189m in capital provided by Japanese ODA (official development assistance) and a consortium of four Japanese contractors, namely Obayashi Corporation, Taisei Corporation, the Kumagai-Kajima consortium and Toa Corporation.