The government also raised the death toll for what has become a point of national mourning and shame to 193. Most of the dead and missing are high school students.
Divers are largely using their hands to feel for remaining bodies as they make their way through a maze of dark cabins, stairwells, storage rooms, lounges and restaurants in the submerged ferry, which flipped upside down as it sank April 16. But they must fight strong currents swirling around the ferry and, once inside, overturned furniture, mattresses and other debris floating in the murky, sediment-heavy waters.
President Park Geun-hye's apology, and the earlier resignation of her prime minister, comes amid rising indignation over claims by the victims' relatives that the government did not do enough to rescue or protect their loved ones.
Park said at a Cabinet meeting at the presidential Blue House that South Korea has "lost many precious lives because of the accident, and I am sorry to the public and am heavy-hearted." She says the government couldn't prevent the accident and "the initial response and remedy were insufficient."
Park had earlier visited a memorial set up in Ansan, the city near Seoul where the high school students are from, to pay her respects to victims. Wearing a black dress and white gloves, she laid flowers at an altar and bowed her head. According to local media, some angry family members of victims shouted at her and demanded an apology. She listened to them for 10 minutes before leaving.
Investigators, meanwhile, are expanding their probe into both the cause of the ship's sinking and the initial response by emergency workers. Prosecutors are also analyzing calls exchanged between crew members of the sunken ferry and the offices of the owner, Chonghaejin Marine Co. Ltd., senior prosecutor Ahn Sang-don said Tuesday.
Multiple crew members on the sinking ferry communicated about seven times by phone with the owner's offices, Ahn said. The first call to the owner was placed at 9:01 a.m. on April 16, just 6 minutes after the ferry reported a distress call to a vessel traffic services center. The last call by a crew member to the employer was made around 9:40 a.m.
South Korean media reports said the captain of the sinking ferry was seeking approval from the CEO of Chonghaejin to be able to evacuate the ship, but Ahn said investigators are still looking into why the calls were made.
Crew members initially asked passengers to stay put and wear life jackets. It is unclear whether an evacuation order was relayed to passengers, although crew members interviewed by The Associated Press said the captain sent an evacuation order.
Of the 475 people believed to have been aboard at the time of the sinking, only 174 people survived, including 22 of the 29 crew members.
The government is making initial plans to eventually salvage the ferry but has indicated it won't do so until search efforts end.
All 15 crew members responsible for the ship's navigation have been arrested, but they haven't been formally charged yet because investigations are still going on. Prosecutors say they were negligent and failed to help passengers in need.
Capt. Lee Joon-seok initially told passengers to stay in their rooms and took half an hour to issue an evacuation order, by which time the ship was tilting too severely for many people to get out. Lee told reporters after his arrest that he withheld the evacuation order because rescuers had yet to arrive and he feared for passengers' safety in the cold, swift water.
Senior prosecutor Yang Jung-jin said that the cause of the sinking could be due to excessive veering, improper stowage of cargo, modifications made to the ship and tidal influence. He said investigators would determine the cause by consulting with experts and using simulations.
Lee reported from Mokpo, South Korea. Associated Press writers Jung-yoon Choi and Foster Klug in Seoul contributed to this report.