Senior prosecutor Yang Jung-jin said the third mate has refused to tell investigators why she made the sharp turn. Prosecutors have not named the third mate, but a fellow crew member identified her as Park Han-kyul.
As he was taken from court in Mokpo on Saturday, the captain explained his decision to wait before ordering an evacuation.
"At the time, the current was very strong, the temperature of the ocean water was cold," Lee told reporters, describing his fear that passengers, even if they were wearing life jackets, could drift away "and face many other difficulties."
He said rescue boats had not yet arrived, and there were no civilian vessels nearby.
Associated Press writers Foster Klug, Youkyung Lee, Jung-yoon Choi and Leon Drouin-Keith in Seoul, South Korea, and Minjeong Hong in Jindo contributed to this report.