South Korea ferry disaster: Unanswered questions
More than 200 people - many of them schoolchildren - remain missing after a ferry sank on Wednesday off South Korea. The BBC looks at some of the questions surrounding the disaster.
Why did the boat sink?
Rescued passengers report hearing a loud thud before the boat began to tilt. This may have been caused by the vessel striking a submerged object such as a rock or a sunken container.
However, the noise may also have been caused by large cargo coming loose aboard the vessel.
The ferry is known to have made a sharp turn shortly before it issued a distress call but is not clear whether this was planned or the result of an external factor, the South Korean Maritime Ministry said.
Experts have suggested that a sharp turn could have caused cargo to shift, potentially destabilising the ship.
According to John Noble, a salvage expert, the speed with which the boat turned on its side - over roughly two hours - could be significant.
He says the ship's compartments appear to have flooded rapidly. This could indicate extensive damage to the hull, or leakage through doors that were meant to be watertight.
Attention is also focusing on who was in command at the time of the incident. Investigators say the ship's third officer was at the helm and it is not clear whether the captain was present.
Was the emergency response appropriate?
The ferry sent out an alarm at around 09:00 local time (00:00 GMT) on Wednesday. Rescue boats and helicopters were on the scene soon after.
"The distress call was put out and the authorities had a structured response," says Bruce Reid, CEO of the International Maritime Rescue Federation, a body that promotes safety at sea. But, he says, it is still too early to come to any conclusions about the effectiveness of the rescue mission.
More questions are being raised about instructions given to passengers.
Several survivors say that the crew ordered them to stay in place when the vessel ran into trouble. Ultimately, only two of the ferry's lifeboats were deployed. Many passengers were rescued after jumping into the sea, wearing lifejackets.
Oh Yong-seok, a crew member, told the Associated Press news agency that the officers initially tried to stabilise the vessel. He says they instructed passengers to put on life-jackets and stay on the ship. The evacuation order was only given after 30 minutes, Mr Oh said, and it may not have reached all the passengers.
However, experts again say it is too early to judge decisions that were made aboard the stricken ship. In emergencies, crews have to think on their feet - and there may be sound reasons for actions that seem questionable from afar.
"There is a requirement that a ship can be evacuated within 90 minutes," says Mr Noble, "but of course that was totally irrelevant in this case."
"In fact, there are no standard regulations," he says. "The crew have to react to what they see. If they do a standard thing for one set of circumstances, it may be the wrong thing for another."
"Traditionally, I'd always say to people, stay with the ship because that's the safest place to be," he says. "But if the ship is sinking and sinking rapidly, then the only answer is to get off."
Passengers and crew say that the speed with which the ship tilted made it harder for them to move around - and for the lifeboats to be deployed.
What hope is there of finding more survivors?
"Time is your greatest enemy," says Mr Reid, warning that the scope for finding survivors shrinks with every passing hour.
The currents that have hampered the rescue divers are equally a danger to any passengers who were swept away from the ship.
"Those currents are quite swift, [which] means that the rescue area would be quite broad," he says.
After previous accidents, passengers have sometimes been rescued from air pockets within sunken vessels. However, there is less chance of surviving for long in cold waters, such as those off the shores of South Korea.
Passengers or crew would be very lucky to find themselves in an air pocket, says Mr Noble. "If they did, they would instinctively make a noise by banging on the metalwork," he says. "And I'm quite sure the rescuers would be listening for that."