South Korean prosecutors are seeking arrest warrants for the captain and two crew members of the ferry that capsized Wednesday, a coastguard official said.
The accident appears to have claimed the lives of hundreds of passengers, many of them schoolchildren.
So far, 28 of the 475 passengers and crew on the Sewol ferry have been officially declared dead and 179 have been rescued.
Experts say it is now unlikely that any of the 268 missing passengers, many of them children from a school on the outskirts of the capital Seoul, will be found alive.
"The joint investigation team of police and prosecutors asked for warrants to arrest three crew, including the captain," the official in the southern coastguard headquarters in Mokpo said.
The request was submitted to the local court, he said, while adding that he was unaware of the precise charges.
The captain and most of his 28 crew managed to escape the ferry, and have been criticised for abandoning the ship when so many were still trapped on board.
Tracking data from the Maritime Ministry showed the ferry made a sharp turn just before sending its first distress signal on Wednesday morning.
Some experts believe such a tight turn could have dislodged the heavy cargo manifest, including more than 150 vehicles, and destabilised the vessel, causing it to list heavily and then capsize.
But others suggested the turn might have been caused by a collision with a rock or other submerged object.
Prosecutors said today that preliminary investigations showed that captain Lee Joon-Seok had handed the helm to his third officer before the ferry capsized.
Mr Lee apologised yesterday to the victims and their relatives, but offered no clear explanation for what caused the ship to go down.
"I feel really sorry for the passengers, victims and families," he said. "I feel ashamed."
It is normal for junior officers to take the helm and the 400km journey from the mainland port of Incheon to the resort island of Jeju was a regular trip in familiar waters.
The incident happened in calm, shallow waters and investigators have focused on the role of the crew as the vessel appeared to have a clean safety record.
Parents of the missing schoolchildren blamed the ship's captain for the tragedy - South Korea's worst maritime accident in 21 years in terms of potential loss of life.
Meanwhile, the coastguard has denied that divers had entered the capsized ferry in a bid to locate any survivors from the accident.
The coastguard also denied television reports that the ship had been fully submerged, over 48 hours after the accident.
Rescuers were pumping air into the dining hall of the vessel, where many of the passengers were expected to be.
Elsewhere, a high school vice-principal rescued from the sinking ferry has been found dead outside a gym where families of the victims were staying.
Police said that Kang Min-gyu, 52, had been missing since yesterday and had died by suicide.
About 340 of the passengers were students and teachers from the Danwon High School in Ansan, an industrial town near Seoul, who were on an outing to the resort island of Jeju.
They account for about 250 of those missing.