Dozens of asylum-seekers on a boat that sank near the Italian coast last month with the loss of 366 lives were raped and tortured in Libya before starting their journey, the police said on Friday.
A group of 130 migrants from Eritrea were held for ransom in the desert, according to testimony from survivors that led to the arrest of a Somali man in Italy accused of being one of the traffickers.
The Somali, Libyan militiamen and Sudanese traffickers all took part in the alleged assaults in a detention centre in Sabha -- an oasis in the desert in southwest Libya -- that prosecutor Maurizio Scalia likened to "a concentration camp".
"They forced us to watch our men being tortured with various methods including batons, electric shocks to the feet. Whoever rebelled was tied up," read the testimony of a 17-year-old Eritrean girl in the investigation, La Repubblica daily reported.
The migrants were forced to pay up to $3,500 (2,600 euros) for their freedom and their onward journey to the Libyan coast and a boat to Italy.
"The women who could not pay were assaulted," the girl was quoted as saying in her criminal charge.
She also described in horrific detail her own sexual assault, saying that the Somali man -- named by police as 34-year-old Elmi Mouhamud Muhidin -- was one of the three men who raped her.
"They threw me on the ground, held me down and poured fuel on my head. It burnt my hair, then my face, then my eyes. Then the three of them raped me without protection. After a quarter of an hour I was beaten and taken back to the house."
Scalia said simply: "All the women in that centre were raped by Somalis and Libyans. It was like a concentration camp."
Migrants from Eritrea and Somalia frequently speak of abuses along their journeys to Europe, particularly in an increasingly lawless Libya following the fall of dictator Moamer Kadhafi.
Their allegations are rarely investigated, however, and migrant rights groups complain that more international action should be taken to ease their plight in the countries they transit through.
The Somali was arrested this week on the remote Italian island of Lampedusa where the October 3 shipwreck happened and has now been flown to Sicily where he faces up to 30 years in prison.
The man arrived on the island on October 25 and had been staying in the local migrant centre, pretending to be one of the refugees, investigators said.
The police said they had not yet worked out why the man had come to Italy but added that he "may have been looking for criminal contacts".
But he was spotted by some of the survivors who remembered him from their incarceration and risked being lynched by them inside the refugee centre.
"He was one of the leaders of the trafficking organisation," a police spokeswoman told AFP.
Another man, the boat's 35-year-old Tunisian captain Khaled Bensalam, was detained immediately after the tragedy on charges of manslaughter.
He was one of the 155 survivors of the tragedy, almost all of whom are still on Lampedusa pending the investigation. Only four minors among the group have been transferred to foster families.
A third man, 47-year-old Palestinian Attour Abdalmenem, has also been arrested on people trafficking charges in a separate investigation for allegedly organising a boat of Syrian migrants that also arrived on Lampedusa, the police said.
Italian authorities have vowed to crack down on the people trafficking rings that have been behind the influx of more than 35,000 asylum-seekers so far this year to the country's coasts.
Most of them come from Eritrea, Somalia and Syria and Italy has asked for the European Union to step up assistance in dealing with the arrivals and countering the criminal networks behind them.