Prosecutors in Sicily say two actions are suspected to have caused a migrant boat to capsize in what is thought to be the Mediterranean's worst migrant disaster.

In a statement Tuesday, Catania's prosecutors said the smuggler captain, 27-year-old Tunisian Mohammed Ali Malek, mistakenly rammed his boat into the Portuguese-flagged merchant ship that had come to its rescue.

And, the prosecutors say, the migrants themselves then shifted position on the boat, which was already off balance due to the collision.

The prosecutors say the death toll is still uncertain, noting that the passengers have spoken of anywhere between 400 and 950 people on board, while the crew of the rescue ship estimated some 850.

Only 24 bodies have been recovered to date, and 28 people survived.