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.
Among them were 90 women – eight of them pregnant – and 52 children.
There were also some Libyan men and women on board.
People-smuggling gangs charge each migrant about $6,000 (£4,578) to get to Italy, $4,000 of which is for the trans-Saharan journey to Libya, according to the Italian aid group L’Abbraccio.
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 Organisation 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.
Special Assistant to the President on Foreign Affairs and the Diaspora Mrs Abike Dabiri said “it is tragic, unfortunate and heartbreaking”.
She said: “We need to step up sensitization and awareness on the dangers of such method of migration.
“The boats being used are now smaller and more dangerous and the people on the other end are not so eager to safe them anymore.
“Don’t forget also, that they pay as much as $4,000 to go on such trips.
“Even if they survive, they only end up as slaves wherever they end up.
“The President Muhammadu Buhari Administration has been working with IOM to bring back volutary returnees.
“About 5,000 were brought back in the last six months, full of regrets and tales of woe that they wished they were never deceived into such trips.
“I condoles with the families of the victims and appeal to our your to stop being victims. It is not worth it, either in the short or long term.”