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 Monday they are trying to find out what the crew of the Philippine-flagged ACX Crystal was doing before reporting the collision to authorities 50 minutes later.

The coast guard initially said the collision occurred at 2:20 a.m. on Saturday 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.

The ACX Crystal collided with the USS Fitzgerald off Japan's coast, killing seven of the destroyer's crew of nearly 300.

A track of the 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 2:20 a.m. to the area near the collision.

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 Monday.

Nanami Meguro, a spokeswoman for NYK Line, the ship's operator, agreed with the revised timing of the collision.

Meguro said the ship was "operating as usual" until the collision at 1:30 a.m., as shown on a ship tracking service that the company uses. She said the ship reported to the coast guard at 2:25 a.m., but she could not provide details about what the ship was doing for nearly an hour.

"Because it was in an emergency, the crewmembers may not have been able to place a call," she said.

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

Associated Press writer Yuri Kageyama contributed to this story.