Five workers remained missing Wednesday after seawater flooded an undersea tunnel being built at JX Nippon Oil & Energy Corp.'s Mizushima oil refinery in Kurashiki, Okayama Prefecture, on Tuesday, local police said.
Police divers resumed their search for the five Wednesday morning but decided later to give priority to removing large quantities of rubble found in a shaft of the tunnel.
The tunnel for a pipeline linking two plants of the refinery has been under construction by Kajima Corp. since 2010 and was due for completion next year.
The flooding occurred as the five workers from subcontractors of Kajima were operating a tunneling shield some 140 meters from a 30-meter-deep shaft.
A shield machine is a protective structure used in excavating through soil too soft or fluid to remain stable during the time it takes to line the tunnel with a support structure.
Kajima officials indicated that a technical problem may have occurred just before the flood. In a phone call prior to the accident, one of the workers asked an engineer who deals with mechanical issues to come to the work site.
Kajima officials denied that a mechanical problem caused the accident.