There are many stories about the truth in "Assassins," but what is the truth about the Sewol Ferry?

‌The Sewol Ferry tragedy is so heartbreaking,
and I hope something like this never happens again.
However, it is also true that since the accident,
a vast amount of national energy has been spent searching for the truth,
which has led to significant division among the people.
That is why I am curious:
What was the truth behind the Sewol Ferry?