South Korean authorities continue investigating what caused a ferry to capsize off the country's southern coast, killing dozens and leaving nearly 300 people missing.
Investigators said Friday they believe the 6,800-ton passenger boat made a sharp turn before listing, or leaning heavily to one side, and eventually capsizing.
However, it is not clear what may have forced the ferry to turn, since weather conditions were relatively calm and the area was seemingly free of large rocks and reefs.
There are also questions about what passengers have described as a loud bang that occurred moments before the ferry began to turn.
Sam Bateman, a retired commodore in the Royal Australian Navy, cautions against making firm conclusions this early in the investigation.
But he tells VOA the problem could be related to the stern, or rear, door of the ship, which allows vehicles and other large cargo to enter.
"I suspect, from the descriptions with this Korean vessel and the talk of this 'bang' business, is that the stern door of the ferry somehow has given way."
If water did, in fact, begin to enter through the stern door, Bateman says it could lead to an effect known as a "free surface," in which water sloshes around the deck, causing the craft to become unstable.
"If you try and carry a big flat oven tray of water or something like that and you sort of tilt it to one side, it becomes off balance very easily. And that's the weakness of ferries, because once you get a bit of water onto a vehicle deck, and the ship's rolling a bit, it's very easy then for the water to create the free surface which creates the listing moment for the ship."
The stern door theory would help explain why such a large ship capsized so quickly. But it does not account for why the door would have given way in the first place.
One possibility is that an item of large cargo, such as one of the 150 cars on the ship, came loose. But Bateman says this seems unlikely, given the reported conditions at the time of the sinking.
"The weather doesn't appear to have been particularly rough. So it shouldn't have been the case like an unsecured vehicle or something like that moving or a container on the vehicle deck. I suspect more some structural failure on the vessel itself."
Bateman, a senior fellow at the S. Rajaratnam School of International Studies in Singapore, does not believe the vessel stuck an uncharted rock, saying investigators would probably have located the rock by now.
The incident is the latest in a series of deadly ferry crashes, which are raising concerns over the safety of passenger boats worldwide.
Bateman says many ferry operators, particularly in developing countries, could improve standards meant to avoid overcrowding. He says passengers should also be given better pre-departure evacuation guidance.
"When you fly on an aircraft, you get that pre-flight briefing about how to put on your life-jacket. And they also talk about the nearest exit route and all that sort of stuff. That rarely happens on a ferry."
When riding a ferry, Bateman suggests passengers make a mental map of how to leave the ship in case of an emergency, locating at least two exits before the ship departs.