Italian prosecutors are investigating the deaths of 26 Nigerian women - most of them teenagers - whose bodies were recovered at sea.
There are suspicions that they may have been sexually abused and murdered as they attempted to cross the Mediterranean.
Five migrants are being questioned in the southern port of Salerno.
A Spanish warship, Cantabria, docked there carrying 375 migrants and the dead women, following several rescues.
Twenty-three of the dead women had been on a rubber boat with 64 other people.
Italian media report that the women's bodies are being kept in a refrigerated section of the warship. Most of them were aged 14-18.
Most of the 375 survivors brought to Salerno were sub-Saharan Africans, from Nigeria, Senegal, Ghana, The Gambia and Sudan, the daily La Repubblica reports.
Many migrants have reported violence, including torture and sexual abuse, by the gangs.
In the year to 1 November, 150,982 migrants arrived in southern Europe by boat from North Africa, the International Organization for Migration (IOM) reports.
Of them, 111,552 (nearly 75%) came via the Central Mediterranean route to Italy. The number who died on that route was 2,639, the IOM says.
The others arrived in Greece, Cyprus or Spain. The total is less than half the 335,158 who arrived in the same period of 2016.
Last year the total for Greece was higher than that for Italy.