As many as 500 people are feared to have died in a shipwreck last week in the Mediterranean Sea, which could be one of the worst tragedies involving refugees in the last 12 months, the United Nations Refugee Agency (UNHCR) said in a statement Wednesday. The agency said that it talked to 41 survivors, who were rescued Saturday by a merchant ship, to find out more details about the overcrowded boat.

The 41 survivors included 37 men, three women and a three-year-old child, and they were taken to Kalamata in Greece, the agency said. The boat submerged in an unknown location between Libya and Italy and those rescued included 23 Somalis, 11 Ethiopians, six Egyptians and a Sudanese. The survivors also told the agency that the refugees were a part of a group of 100 to 200 people, who left from a near Tobruk in Libya on a 30 meter-long boat.

“After several hours at sea, the smugglers in charge of the boat attempted to transfer the passengers to a larger ship carrying hundreds of people in terribly overcrowded conditions,” UNHCR said, adding: “At one point during the transfer, the larger boat capsized and sank.”

The survivors include those who did not board the larger vessel while some swam back to the smaller boat.

The details of the incident varied slightly in a report by the International Organization for Migration (IOM). The organization said that 200 people had departed on small boats, each carrying between 30 to 40 people. The report added that the people were on their way to a larger vessel on higher seas and that over 400 people might have died. According to IOM, about 30 people remained on the smaller boats while 10 more managed swim toward them. The refugees said that each of them paid about $800 to $2,000 to the human traffickers to reach Europe.

“I saw my wife and my two-month old child die at sea, together with my brother-in-law,” a man named Mohamed told IOM, adding: “The boat was going down...down..., all the people died in a matter of minutes. After the shipwreck we drifted at sea for a few days, without food, without anything, I think (sic) I was going to die. When we were rescued we were told them that we wanted to go to Italy, but we have been brought to Greece.”

IOM Athens Chief of Mission Daniel Esdras said in the statement: “The testimonies we gathered are heartbreaking,” adding: “We await further investigations by authorities to better understand what actually happened and find hopefully evidence against criminal smugglers.”

A report by the Associated Press (AP) said that no national authorities have reported the bodies washing ashore

About 380 refugees have died on the Eastern Mediterranean Route between Turkey and Greece while about five refugees have died on the Western Route connecting Morocco and Spain, so far in 2016. The IOM report also said that the organization’s Missing Migrants project estimates that about 1,200 asylum-seekers were killed in 2016 on all Mediterranean routes. Last April, over 1,730 refugees died or went missing, IOM estimated.

The European Commission has signed a deal with Turkey that requires the bloc to settle one Syrian refugee in Turkish camps for each refugee that Greece sends back to Turkey. In exchange, Turkey was promised more funding for refugees, a revival of EU accession talks and quicker visa liberalization. However, rights groups have been highly critical of the deal.

“In the mad dash to start the deportations deal with Turkey, the European Union and Greece tossed rights to the wind, including for people who wanted to seek asylum,” Fred Abrahams, an associate program director at Human Rights Watch, said in a report released Tuesday, adding: “The abusive deportations expose the fundamental flaws in the EU’s expedited mass returns to a country that cannot be considered safe.”