Japan's Transport Safety Board says a fatal collision between a US Aegis destroyer and a Philippine-registered container ship off the Izu Peninsula in 2017 was caused by the destroyer's poor lookout.
The collision occurred in June 2017, killing seven crew members of the destroyer USS Fitzgerald.
The government panel report published on Thursday says the destroyer could not determine the precise position of the container due to inadequate radar adjustment.
It also points out that the crew members on duty failed to keep proper watch as they were focusing on another approaching ship ahead of the container.
The report says the container's navigator officer believed that the destroyer would avoid collision from the two ships' relative positions and did not give out a warning sound signal.
The safety board says that in order to prevent a recurrence of collision, crew members should adjust radar and other navigation instruments and constantly carry out a proper lookout.
It also points to the need for a ship to take action to avoid collision even if it is in a position of not being responsible for avoiding collision.