Japan's Self-Defense Force has begun surveying a lake in the northern prefecture of Aomori where a US fighter jet offloaded 2 external fuel tanks.
The fighter jet from the US Misawa Air Base jettisoned the tanks into Ogawara Lake on Tuesday after its engine caught fire.
Fishing and clam harvesting in the lake has been banned due to fears of water contamination.
Personnel from the Maritime Self-Defense Force, sent at the request of the prefecture's governor, checked the lake surface aboard 3 vessels on Thursday morning, cracking ice as they proceeded.
The crew reported smelling fuel at 4 locations but did not find oil slicks or parts from the plane.
They plan to scan the same area later in the day.