Story highlights The 12 victims were from Nigeria and Ghana, police said

The group of 105 people left Libya, bound for Italy

More than 10,000 people have arrived on Italian shores from Libya since last weekend

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.

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.