A sightseeing boat that sank off Hokkaido a month ago, leaving 14 people dead and 12 missing, dropped back to the seabed at a depth of around 180 meters as a salvage company was attempting to tow it, the Japan Coast Guard said Tuesday, in a setback to efforts to bring it to port for an investigation into the cause of the accident.

The company was scheduled to tow the 19-ton Kazu I to shallow waters in order to load it onto a barge and drain it of seawater for several days before transporting it to the nearby port of Abashiri.

Nippon Salvage Co. had been towing the boat slowly at a depth of around 20 meters after raising it from a depth of some 120 meters on Monday, but the coast guard was told by the company Tuesday morning that the vessel had dropped.