Following a shipwreck that left at least 79 migrants dead, rescuers combed the seas near Greece on Thursday as chances of survivors dropped and concerns that hundreds more—including children—might have perished while trapped inside the packed vessel’s hold increased.

According to reports, the fishing boat that overturned and sank early on Wednesday morning in deep waters approximately 50 miles (80 km) from the southern coastal town of Pylos had between 400 and 750 people on board. According to Greek authorities, 104 people were brought ashore.

As it began to flounder late on Tuesday night, people on the vessel’s crowded outer deck repeatedly turned down attempted assistance from a Greek coast guard boat that was shadowing it, saying they wanted to reach Italy, according to Greek authorities.

“When you are faced with such a situation… you need to be very careful in your actions,” coast guard spokesperson Nikos Alexiou told state broadcaster ERT.

“You cannot carry out a violent diversion on such a vessel with so many people on board… without any sort of cooperation.”

As Greece declared three days of mourning, authorities said it was unclear how many had been aboard. They were investigating an account from a European rescue-support charity that there could have been 750 people on the 20- to 30 metre-long (65- to 100-foot) boat.

The U.N.’s International Organization for Migration said initial reports suggested up to 400 people were aboard.

Citing initial testimonies from survivors, Save the Children charity said around 100 children were believed to be in the vessel’s hold.

“(EU) member states have gone to extraordinary lengths to close off all routes to children and their families seeking safety in Europe. Often their only option is to take dangerous journeys by boat,” said Daniel Gorevan, senior advocacy adviser at the charity.

“The fact that people continue to die in the Mediterranean should be a wakeup call for EU governments,” he said, warning that the Med risked becoming “the deadliest migration route in the world.”

Of the 104 survivors so far transferred by the coast guard to the Greek port city of Kalamata, most were men, authorities said. They revised their overnight death toll to 78 from 79.

‘OUR LAST NIGHT ALIVE’

On Thursday, bodies of the victims were transferred to a cemetery near Athens for DNA tests. The search operation will continue for as long as needed, the coast guard said.

Government sources said chances of retrieving the sunken vessel, which had set off from the Libyan port of Tobruk, were remote, because the area of international waters where the incident occurred is one of the Mediterranean’s deepest.

Aerial pictures released by the Greek coast guard showed dozens of people on the boat’s upper and lower decks looking up, some with arms outstretched, hours before it sank.

Alarm Phone, which operates a trans-European network supporting rescue operations, said it received alerts from people on board a ship in distress off Greece late on Tuesday.

It said it had alerted Greek authorities and spoke to people on the vessel who estimated there were up to 750 people on board and appealed for help, and that the captain had fled on a small boat.

Government officials said that before capsizing and sinking around 2 a.m. on Wednesday, the vessel’s engine stopped and it began veering from side to side.

Independent refugee activist Nawal Soufi said in a Facebook post that she was contacted by migrants aboard the vessel in the early hours of Tuesday, and that she had been in contact with them until 11 p.m.

“The whole time they asked me what they should do and I kept telling them that Greek help would come. In this last call, the man I was talking to expressly told me: ‘I feel that this will be our last night alive,'” she wrote.

Greece is one of the main routes into the European Union for refugees and migrants from the Middle East, Asia and Africa.

Under a conservative government in power until last month, Greece has taken a harder stance on migration, building walled camps and boosting border controls.

The country is currently governed by a caretaker administration pending an election on June 25.

Libya, which has had little stability or security since a NATO-backed uprising in 2011, is a major launching point for those seeking to reach Europe by sea, its people-smuggling networks mainly run by military factions that control coastal areas.

The United Nations has registered more than 20,000 deaths and disappearances in the central Mediterranean since 2014, making it the most dangerous migrant crossing in the world.