The girls, aged 14-18, were believed to have been migrants from Niger and Nigeria who had embarked on the treacherous route to Europe from Libya over the weekend.
Lorena Ciccotti, a senior police official, told CNN on Monday night that autopsies would be carried out on Tuesday and that coroners would be investigating whether the girls had been tortured or sexually abused.
Their bodies were found close to a flimsy rubber dinghy that had all but sunk when rescuers arrived, Ciccotti said.
The girls' rescue was one of four separate rescue operations carried out in the Mediterranean over the weekend. In total, 400 people were brought aboard the Spanish vessel Cantabria before disembarking at the Italian port town of Salerno.
Among them were 90 women and 52 minors, including a week-old-baby, CNN quoted authorities as saying.
On Monday, Italian police arrested two men, an Egyptian and a Libyan accused with human smuggling.
Since the start of this year, 2,839 migrants have died on the central Mediterranean route, according to the International Organization for Migration.
A total of 150,982 migrants have reached European shores, of which around 74 per cent were Italian arrivals.