The disappearance of Yoo Byung-eun and his son has caused a media frenzy in South Korea. Yoo is a member of a church that critics call a cult and have linked to a 1987 mass suicide. Church members deny involvement.
Yoo, 73, was thought to be holed up in a sprawling church compound near Seoul, and there was a tense, days-long stand-off between police and hundreds of church followers, some of whom reportedly threatened to die as martyrs.
But Yoo was not there when church members finally opened the compound to authorities on Wednesday, and some speculated that he might have fled to the home of a church follower.
Prosecutors and police then announced a 50 million won (£30,000) reward for information about Yoo's location, and 30 million won (£17,000) for details about his eldest son.
Yoo, head of the now-defunct predecessor of the ferry's current operator, Chonghaejin, allegedly still controls the company through a complex web of holding companies in which his children and close associates are large shareholders.