SEOUL — What was once a rescue mission now appears increasingly like an operation to recover bodies, with divers managing for the first time late Saturday to break through the windows of a sunken passenger ferry and scour the wreckage.

In the first hours of exploring the vessel, divers retrieved 13 bodies, South Korean coast guard officials said. Previously, bodies had been found only around the ship, as divers had failed for three days to gain access to the Sewol’s interior.

The developments came on a particularly bleak day in which relatives of passengers were asked to give DNA samples, a preparation for identifying bodies that might be found on board. As of Sunday afternoon, 52 had been found dead and about 250 were still missing, according to a briefing by a South Korean coast guard official.

The maritime disaster has prompted national mourning in South Korea, particularly as evidence mounts suggesting mistakes by the crew. Three crew members have been arrested, including the captain .

On Saturday, investigators said they had all but ruled out the possibility that he ferry hit a submerged rock, noting that there was no sign of major external damage to the hull. Investigators have instead raised the possibility that a sudden turn made by the ferry could have caused a shift in cargo, throwing the boat off balance.

At the time of the accident, the ferry was passing through a treacherous channel of small islands and swirling currents; the third mate, not the captain, was steering the boat at that time.

Relatives of passengers have gathered at a gymnasium in Jindo, an island that’s become the staging ground for the disaster.

Those relatives, shown a murky video clip of the underwater rescue effort, responded with fury about the pace of the operation. During the briefing, one man jumped onto a stage in the gymnasium where the relatives had gathered and accosted a coast guard. Another screamed that the ferry’s captain should be brought back to the scene and killed.

The video footage was difficult to make out, showing images of dark-greenish water and the hull of the boat. No bodies were visible.

“What’s the use of showing us this?” one relative asked.

In the days since the ferry went down, hundreds of divers have worked in shifts trying to gain access to the vessel’s chambers. Prior to their breakthrough just before midnight Saturday, they’d encountered a string of problems — from strong currents to poor visibility — and water pressure has prevented them from breaking the ferry’s windows. The divers have used ropes to guide them in their descent, but the ropes have torn on occasion.

On Saturday, divers made it to the third deck of the ferry, where they spotted three bodies. But they were unable to get inside and were reaching a time limit for the period they could spend underwater, said Goh Myeong-seok, a Korean coast guard official.

Complicating matters, search officials said Saturday that oil had begun to spill from the vessel, making it harder to see. Other ships were racing to clean up the oil, but additional oil remains in the ferry and poses a further risk.

“We’ll do our best in search and rescue operation and won’t spare even a second to save precious lives,” Goh said.

In response to the tragedy, some South Korean networks have canceled reality shows and dramas, and almost all film premieres, concerts and galas have been delayed. At least one professional baseball team said there would be no cheerleading at its games.

The ferry sank off the southwestern coast of the Korean Peninsula on Wednesday during a trip from the port city of Incheon to Jeju, a resort island. A total of 476 passengers were on board, most of them teenagers on a high school trip.

The ferry’s captain, Lee Joon-seok, told reporters Saturday after his arraignment that he did not immediately order an evacuation when the vessel ran into trouble because the currents were “very fast around the area of the accident, and the temperature of the water was low.” He also said that at the time of the accident, “there were no rescue ships or fishing boats nearby.”

Lee, wearing a hooded raincoat pulled tight around his face, bowed his head and apologized. But he said that he did ultimately order passengers to abandon the ship — a message some survivors say they never heard.

Lee and two other crew members were arrested early Saturday, with prosecutors saying their actions might have exacerbated the disaster. Lee was allegedly one of the first to abandon the vessel, and prosecutors say he neglected to take necessary steps to safely evacuate passengers.

Of the 174 survivors, all rescued in the first hours of the accident, 76 have been transported to a hospital in Ansan, just south of Seoul. For Ansan, a city of 760,000, the grief has been particularly acute: More than 300 of those aboard attended Danwon High School there.

At the hospital on Saturday, doctors counseled parents on how to prepare their children for a series of funerals.

Seo reported from Ansan