The damaged USS Fitzgerald is towed by a tugboat near the U.S. Naval base in Yokosuka, Japan, southwest of Tokyo, after the destroyer and a Philippine-registered container ship collided off the Izu Peninsula. Seven U.S. sailors died in the incident.

TOKYO — Japan’s coast guard is investigating why it took nearly an hour for a deadly collision between a U.S. Navy destroyer and a container ship to be reported.

A coast guard official said today they are trying to find out what the crew of the Philippine-flagged ACX Crystal was doing before reporting the collision off Japan’s coast to authorities 50 minutes later.

The ACX Crystal collided with the USS Fitzgerald off Japan’s coast, killing seven of the destroyer’s crew of nearly 300. The ships collided early Saturday morning, when the Navy said most of the 300 sailors on board would have been sleeping.

Authorities have declined to speculate on a cause while the crash remains under investigation.

A track of the much-larger container ship’s route by MarineTraffic, a vessel-tracking service, shows it made a sudden turn as if trying to avoid something at about 1:30 a.m., before continuing eastward. It then made a U-turn and returned around 2:30 a.m. to the area near the collision.

The coast guard initially said the collision occurred at 2:20 a.m. because the Philippine ship had reported it at 2:25 a.m. and said it just happened. After interviewing Filipino crewmembers, the coast guard has changed the collision time to 1:30 a.m.

Coast guard official Tetsuya Tanaka said they are trying to resolve what happened during the 50 minutes.

He said officials are planning to get hold of a device with communication records to examine further details of the crash. Japan’s Transport Safety Board also started an accident investigation on Sunday.

Adding to the confusion, a U.S. Navy official said it is sticking with the 2:20 a.m. timing for the crash that he said had been reported by the Fitzgerald.

Asked about the earlier time cited by the coast guard, Navy spokesman Cmdr. Ron Flanders said, “That is not our understanding.” He said any differences would have to be clarified in the investigation.

Coast guard officials are investigating the case as possible professional negligence, but no criminal charges have been pressed so far.