Up to 500 Palestinians, Syrians, Egyptians and Sudanese refugees and migrant workers died last week when their boat sank off the coast of Malta in what appears to be a deliberate ramming by human traffickers amounting to “mass murder,” a United Nations official announced Friday.

Survivors of the tragedy—who reportedly paid $4,000 apiece— told the International Organization for Migration that, before the sinking, they were forced to endure subhuman conditions in which as many as 300 migrants, including up to 100 children, were crammed into a below-deck area, while 200 were held above deck without shade.

Five days into their journey, which embarked from the Egyptian port of Damietta on September 6, the passengers were told by smugglers to move to a smaller and less sea-worthy vessel. At this point, chaos ensued, said the survivors. Many migrants refused to get on the smaller vessel, and some ended up in the water. At a certain point, the smaller boat, manned by smugglers, reportedly rammed the vessel holding hundreds of migrants. “After they hit our boat they waited to be sure that it had sunk completely before leaving. They were laughing,” one survivor told IOM. Only 11 survivors were located and rescued by  authorities in Italy, Malta and Greece.

UN High Commissioner for Human Rights Zeid Ra’ad Al Hussein on Friday slammed what appeared to be a deliberate killing and called for states to “investigate such atrocious crimes, bring the perpetrators to justice, and even more importantly to do more to prevent them from happening in the first place.” He stated, “If the survivors’ accounts are indeed true—and they appear all too credible—we are looking at what amounts to mass murder in the Mediterranean.”