Japan Coast Guard officials suspect human error was the likely cause of a patrol ship running aground in the Sea of Japan off Niigata Prefecture on Wednesday.
The Echigo ran aground in shallow water about 1.1 kilometers northwest of a lighthouse in Kashiwazaki City.
The officials investigating the incident say none of the 33 crewmembers of the ship are injured, and there is no immediate danger of it sinking.
But they also say oil is floating around the vessel, and it is taking on water.
The patrol ship was reportedly approaching the lighthouse, whose light was off at the time, when it ran aground.
The Coast Guard says interviews with crew members suggest the ship was swept away by strong winds while they were preparing to check if the lighthouse was working properly.
Officials are investigating the incident on suspicion of professional negligence endangering passing traffic as the crew may have failed to keep a proper lookout.
They also plan to start a submersible survey by salvage firms on Thursday and will consider how to move the ship.