Days after a fatal collision between a U.S. Navy destroyer and a merchant container ship, little information has emerged to explain how it could have happened. As the families of seven American sailors mourn their losses, here’s a look at what we know, and what we don’t:

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Q. WHAT HAPPENED?

A. Experts generally agree that the Philippine-flagged ACX Crystal, coming from behind, tried to pass the USS Fitzgerald on the right side. Extensive damage to the destroyer’s starboard side and that to the container ship on its port side suggest that, but they say it is too early to determine responsibility. The container ship might have failed to spot the destroyer and rammed into it, or the destroyer somehow might have made a sudden move to the right. There is also a possibility the container ship might have tried to cut in front or in back of the destroyer and accidentally slammed into its side.

“There is almost no mistake the container was behind the destroyer, though it is still premature to decide which ship bumped into the other,” said Tetsuo Kotani, a maritime security expert at the Japan Institute of International Affairs. “The damage to the destroyer’s side suggests the container’s bow slammed into it at a significant speed.”

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Q. WHEN DID IT HAPPEN?

A. Japan’s coast guard initially said the collision occurred at 2:20 a.m. Saturday; the container ship reported the crash at 2:25 a.m. But after interviewing the container ship’s crewmembers, the coast guard now says the collision occurred around 1:30 a.m. The U.S. Navy continues to say the collision happened at 2:20 a.m.

Knowing the timing is essential to make sense of navigation tracking records that show movements of the commercial ship, but not those of the military vessel. The ACX Crystal made a sharp turn at 1:30 a.m., quickly resumed its previous heading and made a U-turn about half an hour later. Those movements are easier to understand if they occurred immediately after the collision than if they occurred before it.

Some experts say the container ship, which is nearly four times the weight of the destroyer, might have continued on without clear understanding of the collision, noticed some irregularity and returned to the collision site, reporting the collision when its crew noticed the destroyer for the first time.

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Q. WHAT IS THE DAMAGE AND WHAT DOES IT SUGGEST?

A. The 8,315-ton destroyer’s starboard side was badly damaged, with a mechanical room and two sleeping compartments destroyed and flooded. Navy officials say the ship also has a big gash on the bottom. Damage to the container ship is concentrated on its bow area, including a sharp horizontal cut across it, scratches and dents on the port side fence and hull. Coast guard officials said the container ship has a speed-increasing bulbous bow that sticks out in front of the ship below the waterline, suggesting that part stabbed the destroyer’s bottom, allowing the seawater to gush in.

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Q. WHY DID THE SHIPS COME SO CLOSE?

A. Two possible causes are a radar failure or negligence by a night watchman — on either ship or both — that might have caused a delay or failure to spot the other ship. Every ship is equipped with radar or other electronic ship tracking device to alert against close encounters, and crewmembers on watch duty provide visual observation. U.S. Navy Vice Adm. Joseph Aucoin said an unspecified number of sailors were on watch duty the night of the collision. The gray paint of the destroyer blends into the darkness and makes it harder to spot at night.

Q. WHAT WENT WRONG?

A. Experts say it could have been a lack of communication between the two ships, or a misunderstanding of the situation as to who should have the right of way. Coast guard officials said they are looking for a recording device on the container ship that could help them determine whether it communicated with the destroyer before the collision. Unlike cars on highways, ships encounter each other from all directions, and confusion can lead to a wrong decision on which side is “give-way” or “stand-on.” Under maritime traffic rules, the ship on the right — in this case, the container ship — gets to proceed, and one to the left is the “give-way” ship.

(AP)