Italian prosecutors in Sicily are seeking an 18-year prison sentence for a Tunisian man who allegedly captained a smugglers' boat that capsized and sank, sending some 800 migrants to their death in the Mediterranean.

The prosecutors on Tuesday also asked the court in Catania to convict the co-defendant, a Syrian who allegedly was the captain's assistant, and sentence him to six years for the April 18, 2015, sinking off Libya.

Twenty-eight people survived, including the defendants, who insist they are migrants, not smugglers. Hundreds of migrants, trapped in the hold, sank with the boat. Italian navy divers have recovered 169 bodies near the wreckage. The navy hopes to raise the wreckage and bring it ashore.

Italian investigators lament that human trafficking masterminds, often based in Libya, elude prosecution.