Five migrants were arrested on Tuesday by Italian police accused of killing fellow passengers aboard a boat in which 30 bodies were found by rescuers earlier this month, Italian media reported.
Italian and Maltese naval crews who rescued more than 560 refugees -- most of them Syrian -- from the ship on July 19 made the grim discovery. The dead, who included a baby, had apparently been overcome by heat and engine fumes having been locked in the hold by the crew.
But Italian police in the Sicilian port of Messina, where the survivors were brought, believe more than 140 people died in shocking circumstances on the crossing from Tunisia.
Police said survivors told them of a massacre as the boat began to founder in rough seas.
According to the witnesses, migrants locked in the hold panicked as the air ran out and forced the door and stormed the bridge. The boat began to sink and after clashes between two groups of passengers on the bridge -- one that wanted to brave the high seas and continue the voyage, and another that wanted to return to Tunisia -- fighting broke out.
Witnessed claimed that in the chaos many people were stabbed while others were thrown overboard in a bid to keep the vessel afloat.
Messina's chief of police estimated that 50 of the migrants were thrown overboard and around 60 were stabbed and then thrown into the sea.
The five men arrested were said to be a 21-year-old Syrian, two Moroccans aged 21 and 25, a Palestinian of 25 and a Saudi aged 32.
Three of the men tried to escape, according to the Italian media, and had bus tickets for Milan in northern Italy in their pockets.
The three Tunisian traffickers, who apparently did nothing to stop the massacre, were arrested by the police on Monday.
The wave of migrants crossing the Mediterranean to Italy -- mostly from Libya -- has dramatically increased in recent weeks with calm summer seas.
Around 80,000 migrants are now believed to have landed in Italy so far this year -- higher than the previous record of 63,000 set in 2011 at the height of the turmoil triggered by the Arab Spring.