Italian police have arrested five men on suspicion of murdering and throwing overboard more than 100 migrants as they attempted to cross from Africa to Europe.

Police said the survivors of the boat journey claimed they witnessed a massacre on board the vessel when their fellow passengers were assaulted and tossed into the sea.

The incident happened when the hull of the boat began to fill with poisonous fumes and people became unable to breathe. Panic ran through those below deck and a fight broke out as people tried to escape and find air.

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To keep the migrants below deck, five men began stabbing and assaulting passengers at random before throwing the passengers overboard, and threatening others not to react or to suffer the same fate, police said.

“In a matter of minutes the heat became unbearable and the air was unbreathable due to the fumes from the motor,” police said in a statement. “Desperation drove the prisoners to force open the door and climb onto deck where the tragedy occurred.”

Police estimated that approximately 60 of the migrants were assaulted and thrown into the sea, while around 50 are thought to have been thrown straight out of the boat.

The bodies said to be thrown overboard have not been recovered.

The boat, which was thought to have held up to 600 migrants on board, the majority of whom were believed to be Syrian, was intercepted by Italian and Maltese authorities on Saturday, at which point 18 people were found dead in a tangled mass of bodies in the hull.

The dead were thought to have suffocated from the fumes emitted from the old boat’s engine, while one other person died on the way to hospital.

The five men; two Moroccans, a Saudi Arabian, a Syrian and a Palestinian, have been accused of carrying out the murders. They are expected to be assigned lawyers. Two more men have been charged for people smuggling.