People traffickers tortured and raped African migrants whose boat later sank off Lampedusa with the loss of more than 360 lives, Italian police say.The police have arrested a Somali man on Lampedusa accused of committing crimes with the armed gang.
Most of the victims on 3 October were Eritreans and Somalis. Their fishing boat capsized near Lampedusa, a tiny Italian island off North Africa.
The migrants each paid thousands of dollars to the gang, police said.
The gang moves people across the Sahara to Libya, where they are detained in a camp until they pay at least $3,000 (£1,866), police said.
The migrants have told the police that torture and rape goes on at the camp, the BBC's Alan Johnston reports from Rome.
Once the migrants make their payment, they are handed on to another organisation which - in return for more money - arranges a boat journey to Europe.
Motives unclear The alleged captain of the Lampedusa boat, a Tunisian man named Khaled Bensalam, is being held in Sicily.
The 24-year-old Somali suspect was arrested after arriving on Lampedusa on 25 October with a group of migrants.
As he entered a migrant reception centre, he was attacked by some survivors of the disaster, police said. There were 155 survivors of the tragedy.
The migrants said they recognised him as one of the leaders of the group that had arranged their long and disastrous journey. He is now facing charges relating to kidnapping, sexual violence and people smuggling.
It is not yet clear why the suspect made the journey to Italy and put himself at risk of being identified by migrants that he allegedly mistreated. But the police say he might have been fleeing tensions within his people-smuggling network.
Another theory is that he may have sought to consolidate links with criminal partners in Italy. Or he might have simply decided that he too wanted to make a new life in Europe, our correspondent says.
Italian media say the Somali man's arrest followed investigations by Sicilian police and anti-Mafia police based in Rome. He is now being questioned in Sicily.