A gas explosion occurred May 24 inside a tunnel under construction in Niigata Prefecture, killing four workers. It is known that the workers were not carrying explosive gas detectors. The police must carry out a thorough investigation of the contractor, Sato Kogyo Co.

They also should examine whether the infrastructure and transport ministry had properly advised the company and its subcontractors concerning the prevention of an explosion.

The construction of the 2.8 km-long Hakka-toge (Hakka mountain pass) Tunnel started in the spring of 2007 as part of National Highway No. 253 linking Minami Uonuma City and Tokamachi City, both in Niigata Prefecture.