Rome ( CNN ) Muslims who were among migrants trying to get from Libya to Italy in a boat this week threw 12 fellow passengers overboard -- killing them -- because the 12 were Christians , Italian police said Thursday . Italian authorities have arrested 15 people on suspicion of murdering the Christians at sea , police in Palermo , Sicily , said . Why migrants are dying to get to Italy . The original group of 105 people left Libya on Tuesday in a rubber boat . Sometime during the trip north across the Mediterranean Sea , the alleged assailants -- Muslims from the Ivory Coast , Mali and Senegal -- threw the 12 overboard , police said . Other people on the voyage told police that they themselves were spared `` because they strongly opposed the drowning attempt and formed a human chain , '' Palermo police said . The boat was intercepted by an Italian navy vessel , which transferred the passengers to a Panamanian-flagged ship . That ship docked in Palermo on Wednesday , after which the arrests were made , police said . ' I enter Europe or I die ' : Desperate migrants rescued this week off Italy . The 12 who died were from Nigeria and Ghana , police said . Thousands of people each year make the dangerous sea journey from North Africa to Europe 's Mediterranean coast , often aboard vessels poorly equipped for the trip . Many of them attempt the voyage to flee war and poverty in Africa and the Middle East . More than 10,000 people have arrived on Italian shores from Libya since last weekend alone , according to the Italian coast guard . Many die each year while attempting the voyage , often when boats capsized . Last year at least 3,200 died trying to make the trip . Since 2000 , according to the International Organization for Migration , almost 22,000 people have died fleeing across the Mediterranean . The IOM reported Thursday the latest boat to sink in trying to make the journey . Only four people survived from the original 45 on board , bringing the estimated death toll so far this year close to a thousand . CNN 's Hada Messia reported from Rome , and CNN 's Livia Borghese reported from Augusta , Sicily . CNN 's Jason Hanna wrote in Atlanta . CNN 's Ben Wedeman contributed to this report .
The alleged victims were from Nigeria and Libya , prosecutors said . The group of 105 victims left Libya , bound for Italy . More than 10,000 victims have arrived on Italian flights from Libya since last weekend .