South Korean President Park Geun-hye has condemned the conduct of some of the crew of the ferry that sank last week, calling it "akin to murder".
Ms Park said that those to blame would have to take "criminal and civil" responsibility for their actions.
Divers are continuing to recover bodies from the ferry, as they gain access to more of the submerged hull.
The death toll now stands at 64, with 238 people still missing, most of them students from a school near Seoul.
Bodies are being brought two or three at a time back to Jindo, a southern island close to where the ferry sank.
Police, meanwhile, have been given access to hundreds of messages sent by passengers and crew so they can construct a detailed chronology of the ferry's last hour.
Ms Park, whose government has faced criticism over its initial response to the disaster, told aides that the actions of the captain and some of the crew "were utterly incomprehensible, unacceptable and tantamount to murder", the presidential office said.
"The captain did not comply with passenger evacuation orders from the vessel traffic service... and escaped ahead of others while telling passengers to keep their seats. This is something that is never imaginable legally or ethically," she said.
Those who had broken the law or "abandoned their responsibilities" would be held to account regardless of rank, she said.
A total of 174 passengers were rescued from the Sewol, which capsized as it sailed from Incheon in the north-west to the southern island of Jeju.
But there were 476 people on board - including 339 children and teachers on a school trip. Many were trapped inside the ship as it listed to one side and then sank.
Investigations are focusing on whether the vessel took too sharp a turn - perhaps destabilising the vessel - before it started listing and whether an earlier evacuation order could have saved lives.
Details of the panic and indecision on the bridge emerged on Sunday, when the coastguard released a transcript of the last communications between the crew and controllers.
In the transcript, a crew member repeatedly asks if vessels are on hand to rescue passengers if evacuation is ordered.
The captain, Lee Joon-seok, has said he delayed the move for fear people would drift away.
Mr Lee, 69, was not on the bridge when the ferry began listing. It was steered by a third mate who had never navigated the waters where the accident occurred, prosecutors said on Saturday.
The captain and two other crew members have been charged with negligence of duty and violation of maritime law.
Four more crew members were reported to have been detained on Monday over allegations they failed to protect passengers.
Investigators had also banned the head of ferry operator Chonghaejin Marine and its largest shareholder from leaving the country, Yonhap said.
It has since emerged that Mr Lee appeared in a promotional video for the journey four years ago, describing the ferry journey as safe as long as the passengers followed the crew's instructions.
A coastguard spokesman said divers on Monday were focusing on the third and fourth decks where cabins were located in their search for the missing.
"We have also opened a route leading to a dining hall, and will try to enter that area," AFP news agency quoted the spokesman as saying.
Over the weekend, there were angry confrontations between relatives of those on board and police, after a group began a protest march.
The relatives say they want more information both about what happened and about how soon the remains of their loved ones can be recovered.
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