The Sewol had 476 passengers and crew on board, including 339 children and teachers. It had an experienced captain, was navigating well-known waters and had passed its annual inspections since it was bought second hand in 2012 by Chonghaejin Marine Co. Ltd.
But prosecutors believe the vessel capsized after turning at too high a speed. Sixty-four people are known to have died and 238 are missing, presumed dead, mostly children.
In an arrest document, the captain was charged with undertaking an "excessive change of course without slowing down" while traversing a channel off South Korea's southwestern tip. He was also charged with negligence in evacuating passengers.
The sequence of known events however offers little clarity on why the ship should have turned at speed.
According to fishermen and others who navigate the tidal waters around Jindo island where the Sewol started to sink last Wednesday, the route followed by the ferry from the port of Incheon to Jeju was regularly used by ferries and larger vessels such as oil tankers.
There were few navigation risks in the main channel, they said. The Korean Meteorological Association said there was a 0.5 metre swell. It was cloudy, but there was no fog.
"The shores of the islands nearby drop straight down," said Hwang Wan-soon, captain of a 9.77 ton fishing boat who has 40 years' experience sailing in the area.
Han Sang-sik, head of the Jindo office of the Dadohae Haesang National Park - an area covering 1,700 islets - said the channel in the spot where the Sewol sank was 37-43 metres (122-142 feet) deep and the channel itself was 3 km (two miles) wide, offering plenty of room for manoeuvre.
"People living in nearby islands say fishing boats tend to avoid the area at full moon as the current is especially strong at that time," he said. There was a full moon the night before the accident.
But that should not have been an issue for the Sewol, with a gross tonnage of almost 7,000 tonnes and one of the largest passenger ferries in operation in Korean waters.
The accident happened when the ship made an abrupt turn and started to list sharply. That's when the ship was suspected of going too fast. It was not clear if the sudden change of direction was one of two scheduled course changes to navigate the channel.
The vessel was bought second-hand from Japan's A-Line shipping company, reconfigured and expanded and was subject to a five-month testing period by the Korea Register of Shipping between October 2012 and February 2013 before it entered service on March 15, 2013.
It was subjected to an annual check again in February 2014.
Jung Young-jun, executive vice president of the survey division at Korean Register of Shipping, said the capacity expansion from 800 to 900 passengers was completed in accordance with "national laws and Korean Register requirements".
Evidence given to prosecutors and from the dialogue between the ship and control centres say cargo started to shift on the foredeck, but it was not clear whether that caused the boat to heel after the turn or was a consequence of the heeling.
Moon Ki-han, an executive at Uryeon (Union Transport Co.), the firm that undertook supervision of cargo loading, told Reuters there were 105 containers onboard.
Of these, 45 were loaded on to the front deck and 60 into the lower decks. In total, the ship was carrying 3,600 tonnes of cargo including containers, vehicles and other goods.
One of the lines of investigation is that a sharp turn could have caused the cargo to shift, which in turn could have contributed to the swift capsizing of the ship. Investigators have seized the records of Uryeon and the ship owner.
The captain was not on the bridge at the time of the initial listing - not unusual on a 13.5-hour voyage. Navigation was the responsibility of a 26-year-old third mate on her first passage in charge through these waters.
Transcripts of conversations between the bridge of the Sewol and land-based shipping control indicate that the ferry initially contacted the control in Jeju island about 56 miles (90 km) away and not the nearest centre on nearby Jindo island when it started to list.
It is normal procedure to use the destination control as the centre except in the case of an emergency and it is not known why the radio operator continued to contact Jeju.
At 08:55 a.m on April 16, while traversing the Maenngol Channel, the Sewol told Jeju Vessel Traffic Service: "This ship is in danger. This ship is listing."
A minute later the ship radioed: "This ship is starting to capsize. Please come quickly." By 9 a.m, it had reported the ship had tilted leftward. "Containers have fallen," it said.
At 0905 a.m., Jeju control contacted the Jindo station, which was in a position to call for assistance from local vessels.
Testimony from witnesses and the transcript of the conversation between the ship's bridge and the control centre in Jindo shows that the crew of the ship did not believe there were enough rescue vessels in the area to save the passengers.
ONLY TWO LIFEBOATS DEPLOYED
It was not known why no more than two of the Sewol's 46 lifeboats were deployed, nor why the captain and crew did not follow emergency procedures laid down in their own manual.
"In case of fire, collision, retreat, terror and any other emergency, the crew should follow the captain's command; make an announcement of an evacuation and (location of) the emergency exit doors; guide (passengers) to escape routes in the area of their responsibility," the manual says, adding that lifeboats and lifebuoys should be checked monthly.
Witness testimony and the conclusion of prosecutors investigating the sinking show that the captain and some of the crew evacuated the ship before most of the passengers, who were instructed in some cases to remain where they were.
Later transcripts show the decision to evacuate being placed in the hands of the captain and under the standing rules of the ship, it was the captain's final decision.
"We don't know the situation over there, so captain you should make a final decision about whether or not to evacuate the ship," the control on the island of Jindo said.