Investigators have only begun their search for answers into what caused a Philippine container ship to collide with a U.S. Navy destroyer over the weekend, which led to the death of seven U.S. sailors. One of the biggest questions is why it took the container ship’s crew nearly an hour to report the collision.

Each country is leading its own investigation into the crash, and neither would speculate at this point who might have been at fault. The container ship, the ACX Crystal, is about three times the size of the destroyer, and location-tracking data shows it made a sudden U-turn at about 1:30 a.m. This is around the time the Japanese coast guard says the vessel collided with the U.S. destroyer. The ship then made another U-turn an hour later, about the time it reported the collision, and circled back toward the site of the crash. A spokeswoman for the container ship’s operator, NYK Line, agreed with this timeline of events, and said the reason for the delay could have come from confusion after the crash. “Because it was in an emergency,” Nanami Meguro told the Associated Press, “the crew members may not have been able to place a call.”

The Japanese coast guard said it’s still trying to understand what happened during this hour, and investigators have asked for communication records. They have said the case may involve possible professional negligence, but it’s not clear which side this is directed toward. Japan’s Transport Safety Board has started its own investigation, as has the U.S. military, which, because of an agreement signed with Japan, could lead the inquiry.

What’s quickly added to the confusion is the U.S. Navy has ignored the Japanese coast guard’s latest timeline. Instead , it says the collision occurred at 2:20 a.m., a full hour later. The USS Fitzgerald was damaged on the right side—the starboard side—and maritime law requires ships to give way to the vessel on their starboard side. When asked about this Sunday, the Seventh Fleet commander, Vice Admiral Joseph Aucoin, declined to answer. The Navy could also decide to declare as classified information radar and communications from the destroyer, which would also hamper the investigation.

Damage to the ship was substantial, and the collision ripped a hole in the destroyer’s hull beneath the waterline. Much of the ship’s crew of 300 was asleep during the crash, and water flooded a radio room, a machine room, and two crew compartments. The first accounts of what happened after the crash indicate there was widespread confusion, and the mother of a sailor aboard the ship told the Associated Press that some sailors ran to man the guns, believing the ship was under attack. Seven sailors died, and two others were flown by helicopter to hospital and later released.

Reuters reported that naval historians believe this is one of the more deadly warship crashes to happen in peacetime in the last 50 years. In 1964, the HMAS Melbourne aircraft carrier collided with a destroyer and killed 82 crew members.