"It was the third officer who was in command of steering the ship when the accident took place," state prosecutor Park Jae-Eok said.
"The captain was not in command when the accident took place," Park said told a press briefing on preliminary findings of the investigation into the disaster.
Captain Lee Joon-Seok, who has become a focus of public anger for escaping the ferry while hundreds of others were trapped, was "in the back", Park said, without elaborating.
Most experts believe the ship either hit a rock or made a sharp turn that shifted its heavy cargo consignment -- including more than 150 vehicles -- and caused it to list and capsize on Wednesday morning.
"Whether or not they took a drastic turnaround ... is under investigation," Park said, adding that they were also looking at the possibility of steering or other mechanical failure.
Chief prosecutor Lee Seong-Yoon said there was "no limit" to the range of the investigation and promised that the precise cause of the sinking would be revealed.
"We will make sure... those responsible are sternly held accountable," Lee said.