It began with blips on a radar screen, 12 miles off the Libyan coast. As the rescuers approached, they found overloaded wooden vessels and rafts that evoked scenes of the slave trade.

Hundreds of African migrants were crammed into boats headed for Italy. More than two dozen people were dead in one boat alone, asphyxiated from the crush aboard. In other boats, bodies were splayed on the floorboards, forcing survivors to clamber over the corpses of their fellow voyagers.