Rescuers scoured seas off Greece on Thursday in a massive search operation, as hopes dwindled of finding survivors of a shipwreck that killed at least 78 migrants in one of Europe’s deadliest such disasters in recent years.

Reports suggested hundreds of people had packed the fishing boat that capsized and sank early on Wednesday in deep waters about 50 miles (80 km) from the southern coastal town of Pylos, while being shadowed by the Greek coast guard.

As dawn broke on Thursday, a coast guard vessel sailed into the nearby port city of Kalamata, transferring victims. After an official count, authorities revised the death toll to 78 from 79. They said 104 people were rescued.

They said it was unclear how many had been aboard, but were investigating one 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 UN’s International Organisation for Migration said initial reports suggested up to 400 people were aboard.

News portal Proto Thema and Skai TV reported that, according to witnesses, mainly women and children were in the vessel’s hold.

Government officials said migrants on the boat, which had set off from the Libyan port of Tobruk, had repeatedly refused offers of help from Greek authorities.

“It was a fishing boat packed with people who refused our assistance because they wanted to go to Italy,” coast guard spokesperson Nikos Alexiou told broadcaster Skai TV.

“We stayed beside it in case it needed our assistance which they had refused.”

No engine, no captain

The search operation was due to continue until at least Friday morning, according to government sources. Chances of retrieving the sunken vessel were remote, they said, because the area of international waters where the incident occurred is one of the deepest in the Mediterranean.

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.

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, authorities have taken a harder stance on migration, building walled camps and boosting border controls.

Greece’s caretaker administration, in power between an inconclusive election on May 21 and new elections on June 25, declared three days of national mourning.

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.

People-smuggling networks are 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.