Before leaving Nigeria, they were subject to a ritual that involved taking an oath to not speak out or flee, and to pay back the cost of getting to Europe, police said in a statement.
Police in Italy estimated based on money transfers traced during the investigation that the suspected traffickers made upwards of 1.2 million euros (about $1.4 million.) The suspects include five men and five women, with four more people sought.
Police said the organizers of the ring were careful that the women not pay their “madams” directly in Italy. Rather they would wire the money to either the recruiter or their families in Nigeria, who would then pay off the local recruiters.
Details of how the network operated were provided to investigators in April 2017 by a woman, identified only as Giuly, who was rescued at sea along with 433 migrants by the humanitarian group SOS Mediterranee, police said.
In all, police said they identified 20 victims of the ring, which had divisions in Nigeria, Libya, Italy and other European countries.
For years, smugglers have taken advantage of Libya's lawlessness to organize migrant crossings to Italy. Italy's crackdown on migrants and the coronavirus pandemic have slowed the number making it to the country in the last two years, with 5,472 arriving so far this year, according to Interior Ministry data.