By Yoo Seungki

SEOUL, April 13 (Xinhua) -- A South Korean documentary film, titled "Intention," opened in local theaters nationwide on Thursday to give the most convincing clue, to date, to the cause of the country's worst maritime disaster in decades.

"Intention," directed by Kim Ji-young and produced by renowned journalist Kim Ou-joon, focused on matching the automatic identification system (AIS) track data with testimonies of survivors to provide a clue to only two questions the bereaved families have long asked: Why the vessel sank and why the victims were not rescued.

On April 16, 2014, the Sewol ferry capsized and sank in waters off the southwestern island of Jindo. A paroxysm of grief swept over the entire nation as victims were mostly high school students on a field trip to the southern resort island of Jeju.

Among the 476 passengers on board the ill-fated ship, only 172 were rescued. A whopping 299 have been confirmed dead, with five others still unaccounted for.

It took the director about three and a half years to complete the movie as he had to collect and analyze a massive amount of the AIS raw data from the Coast Guard and radar logs from the Navy, before cross-checking the objective data with testimonies from survivors.

The AIS is like a car black box offering a real-time information on the car's speed and location, but the AIS is shared by other ships and the maritime authorities to avoid collision and control traffic at sea.

The film traces back to the Sewol's voyage from April 15, when it left a port in Incheon, west of Seoul, to the fatal day when it sank. After a long studying, the documentary found a bizarre track data of the vessel before its sinking.

The passenger ship repeated sharp turns when it sailed near small islands in the West Sea scattered with a number of islets. When it passed very close to the last islet before its sinking, the vessel made a left turn that is not physically possible in the real world.

The breakthrough came after the 6,825-ton vessel was brought to the surface and moved to a port in Mokpo, some 410 km south of Seoul, in March 2017. From inside the salvaged ship, the footage of black boxes of cars parked in a different part of the vessel was restored, giving a powerful clue to the cause of the sinking.

According to the footage showing the inside of the ship, the Sewol listed violently from an angle of 15 degrees to 45 degrees within a second. A physicist said in the film that there is no way to explain the fast tilting except for an external force.

If the Sewol were to make a sharp turn because of the overloading and the poor helmsmanship, it must have taken about 40 seconds to tilt from the angle of 15 degrees to 45 degrees, the director said at a preview event for foreign correspondents in Seoul on Thursday, referring to the simulation assisted by scientists.

According to the recent government announcement, it was groundless to explain the Sewol sinking with overloading and inexperienced steersmanship. There had been no such incident in the past though other ships sailed with more cargo being loaded than the Sewol did.

The director and the producer set up the most convincing hypothesis, to date, on why the ship sank at the sea at the day. It can naturally give a convincing clue to the question the bereaved families have long asked: why they did not rescue the victims.

The film ended with proposing the hypothesis and raising many questions, while intentionally not giving an answer to the question: who was behind it.

"(The film) was produced to make many people ask why ... The rest of it is held responsible for the government and state agencies," said the producer.

President Moon Jae-in, who took office in May last year, vowed to get to the bottom of the maritime disaster. The government launched a series of probes into suspicions surrounding the tragedy.

State prosecutors indicted former aides to the impeached President Park Geun-hye last month on charges of obstructing the past probe by a special investigation panel.

Monday will mark the fourth anniversary of the Sewol ferry calamity. The bereaved families and volunteers still stay in the Gwanghwamun square, asking for a clear, verified answer to the two questions they have long asked.