The United Nations refugee agency said on Wednesday that 500 people may have died in the choppy waters of the Mediterranean last week, when a large boat packed with migrants from Africa and the Middle East capsized in an unknown location between Libya and Italy.

If confirmed, it would be the worst humanitarian calamity in Europe’s migrant crisis since more than 800 people died last April near Libyan shores as they tried to reach Italy.

The agency based its findings on interviews with 41 survivors of the shipwreck, although it was not able to verify the episode independently. The migrants — 23 Somalis, 11 Ethiopians, six Egyptians and a Sudanese — were picked up by a merchant ship near Greece on April 16 after days of drifting at sea. They were transferred to a migrant camp in Kalamata, a city on the Greek mainland.

Their stories helped lift a cloud of confusion about the episode ever since rumors of the sinking emerged over the weekend. But they did not resolve the questions of where the ship went down or what the ultimate death toll may be. No national coast guards have reported finding the boat.