The Yokohama Local Marine Accidents Inquiry Commissioners' Office asked authorities Aug. 21 to investigate the July 2 accident in which a Panamanian-registered supertanker ran aground in Tokyo Bay, resulting in a massive spill of crude oil.
The Yokohama Marine Accidents Inquiry Agency was asked to investigate the cause of the accident involving the Diamond Grace, a 259,999-ton Japanese-owned tanker, the commissioners' office said.
The agency can suspend licenses or issue warnings to ship captains if they find them responsible for accidents at sea.
The Diamond Grace leaked an estimated 1,550 kiloliters of crude oil between Yokohama and Kawasaki, after scraping the seabed near Yokohama's Honmoku Pier.
The Yokohama Maritime Safety Headquarters will file a complaint with prosecutors accusing the captain, Hidenori Tsunematsu, of professional negligence, headquarters officials said.