Our investigation began in Benin City, capital of Edo State. Almost everyone we spoke to has at least a friend or a family member in Europe. More than three-quarters of illegal prostitutes in Italy are from this region. Due to high unemployment among the youths in Edo state, many young women see fewer prospects here. They seek for a better life in Europe instead, not fully aware of the dangers.

Catholic Sister, Bibiana Emenaha, has tried for years to warn young Nigerian women before they ended up in Europe. "Many are lured with false promises," she told us. The traffickers promise jobs such as babysitting or hair dressing, but that quickly turn out to be a lie. Once the young women are in Europe, they end up on the streets.

After long negotiations, a trafficker agreed to an interview with us. He called himself Steve and claimed he has already transported more than 100 Nigerians all the way to Libya. He wouldn’t speak about the people behind his business. He said he was simply a service provider. "The people here in Edo State are greedy. They are willing to do anything for a better life," Steve said.

For 600 euros ($666) per person, Steve organizes the journey from Nigeria to Libya. "Most people know how dangerous the journey is through the Sahara," the human smuggler told us. Many people die very often along the way. "That is the risk," Steve said, who brings the migrants personally to Agadez in Niger. A colleague then takes over from there.

The desert town of Agadez was the most dangerous part of our research trip. The town thrives on human and drug trafficking and foreigners are often kidnapped for ransom. We could only move around with armed guards and had to wear traditional head cover to be less visible.

Like many others in the desert town, Omar Ibrahim Omar, the Sultan of Agadez, sees human trafficking as a problem that cannot be solved in Agadez. He is asking for more money from the international community. His argument: If Europe does not want more migrants to keep coming through the Mediterranean Sea, Europe should give more support to Niger.

For months now, several trucks with migrants from Agadez set out every Monday shortly before sunset towards the north. The crisis in Libya has contributed to human traffickers being able to reach the Mediterranean Sea without the usual controls. And we soon learned that the authorities here in Niger have little interests in their activities.

Many of the migrants from Nigeria land on the streets in Italy. Social worker Lisa Bertini works with foreign prostitutes. "They are coming more and more," she told us. According to official figures, about 1,000 Nigerians went to Italy across the Mediterranean in 2014. In 2015, the figure climbed to 4,000. "And the girls are getting younger," the social worker said.

With help from a Nigerian colleague, we discovered an alleged "Madam" in northern Italy. A Nigerian host in Italy is referred to as "Madam," she is at the top of a smaller trafficking network. The madam we found lived in a suburb of Florence and one victim made serious accusations against the her: "She has been beating us and forced us into prostitution," the victim said.

As we confronted the supposed "Madam" about the accusations, she admited accommodating six young Nigerian women in her house, but denied forcing them into prostitution: "It's just something young Nigerians here do." After our interview, we handed our research to the Italian public prosecutor's office.

Sister Monika Uchikwe has long been criticizing the inactivity of the Italian authorities. For eight years, she has cared for victims of human trafficking. She explained in rage as we asked about the customers. The men always want cheap satisfaction – sex with a Nigerian woman on the streets costs only 10 euros. "Without this possibility, this problem would not exist," she said.