Sandra knew there was always a chance that her clients would kill her.

For three years, she was forced to work as a prostitute on the streets of Moscow, repaying a $45,000 debt to the trafficker who brought her from Nigeria.

"There were five of them," she recalls of one occasion. "They were brutal, they beat me up, they brought out a knife and tried to stab me."

Instead, they pushed her out of the two-story window for not submitting.

Often times, there were more men -- 10, 15, 20 per call.

"They might even kill you if you try to defend yourself," she says. "That's the reason why it is very horrible. And in that process most Nigerian girls lose their life, because not every girl can withstand the pressure of 10 men."

Sandra, not her real name, is one of tens of thousands of Nigerian women who have been trafficked into Europe for sexual exploitation. And many of those women come from a single city.

For decades, Benin City, the capital of Edo State in southern Nigeria, has been tied to trafficking to Europe. Here, a potent mix of poverty and spiritualism drives thousands of young women to make the dangerous journey.

A marketplace in Benin City.

Along its often unpaved, mud-ridden streets there are houses with wide gates and high walls. These belong to the families with a relation who has "made it," says Roland Nwoha, a local NGO worker who has devoted his career to stopping the trade. "Almost every family has a contact in Europe."

Organizations like Nwoha's help educate people about the risks. But he says these few stories of success continue to be a powerful motivator in a city where so many live in desperate conditions.

And in Benin City, the push to leave comes from every direction.

Trapped by fear

Sandra says she was convinced to go by a man she met at church, who said he was an assistant pastor.

She says he told her he had a vision from God that she traveled overseas, that his sister in Russia could get a job in a hair salon. For added insurance, the man had given the items she left behind to a traditional priest.

"We always have had this belief that your future lies in the hand of God," says Nwoha. "Religious leaders, both the traditional and the Christian, are capitalizing on this."

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Like so many, Sandra feared the juju -- traditional witchcraft -- as much as she trusted her friend.

Her trafficker took much more than just her passport. "My pants, my bra, the hair from my head, the armpit and my private parts," she says.

The items were for a juju oath, so powerful, a local priest said, that no one dares break it.

For Sandra, it bound her to her home thousands of miles away in Benin City, and the assistant pastor that convinced her to go.

"I saw it with my own eyes. It's like a danger to weak girls, especially when it has to do with sensitive parts of your body."

She believed that her passage to Europe would cost her no more than $2,000. She ended up owing her trafficker $45,000.

The average debt for girls trafficked from Nigeria is around $25,000, but it can be as much as $60,000. None of them have any idea that they will owe these extortionate amounts. The debt, and the fear of juju, keeps them trapped.

Sea of misery

Sandra's journey took her through Lagos and then an onward flight to Europe.

But increasingly the trafficking trade is flowing through the lawlessness of Libya and across the Mediterranean where, according to the UN's International Organization for Migration (IOM), over the past three years there has been a 600 percent rise in the number of potential sex trafficking victims arriving into Italy by sea.

The IOM estimates 80 percent are from Nigeria . The majority are from Benin City.

"When the Europeans started their search and rescue operations, many people in Benin said, 'the road has opened, once you get on the boats you will be rescued," says Nwoha.

But just last month, the bodies of 26 Nigerian women were recovered from the Mediterranean in a single day, bringing this year's total number of migrant deaths in that sea to at least 3,000.

Often, the journey ends in tragedy. More often, the tragedy happens in Libya.

Ede's story

Physically, 28-year-old Ede is finally free, but the pain of what she endured is still raw.

"He used to hurt me, apart from work," she says of the man who purchased her. She was sold into sexual slavery in Libya as she tried to make her way to Europe.

"That is how they do there," says Ede, "When you finish paying your money [to your captor], if you are staying with a wicked somebody, they will sell you to another people so you start all over again."

She was freed after a police raid and eventually deported to Nigeria.

Main migration routes from Nigeria to Europe

Now, back in Benin City, she sits next to 18-year-old Jennifer, who is too traumatized to talk. They are recent rescues, kept in a safe house run by the National Agency for Prohibition of Trafficking in Persons (NAPTIP).

"Especially they hate us, we Nigerians ... they don't even want to hear anything concerning Nigerians," Ede remembers. "They treated us like a slave, as if we are nothing. So we went through a lot there."

Outside, the house is a non-descript, high walled compound, just like the others in the neighborhood.

Inside, the young women sit in a dark living room, where the hum of an overhead fan, and the Nigerian soap opera on TV are the few comforts in this temporary home as they wait for their cases to be investigated and to be reunited with their families.

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Reducing demand

But few cases end up in court. Fewer still end in convictions.

According the US State Department's latest Trafficking In Persons report , last year NAPTIP reported 654 investigations, with 23 convictions for trafficking offenses.

"We're prosecuting the small fries in Nigeria," says Julie Okah-Donli, director general of NAPTIP. "Absolutely the number one problem is the inability of destination countries to clamp down on their own criminal networks.

"We've looked at the root causes in Nigeria without addressing the root causes in the destination countries," she says. "What is being done to reduce the demand for this crime?"

Sandra's case is one of the rare prosecutions. Her trafficker was arrested, as was his sister, who was Sandra's "madam" in Russia, pimping her out to clients. They are both awaiting trial.

"When I was in Russia I said to myself, if I get back to Nigeria alive I will expose her," says Sandra. "She is not going to go unpunished. The wicked don't have any place here, they have to face the law."

Her former church admits her trafficker was a member of the congregation but denies that he was an assistant pastor.

Pastor Etinosa Osiomwanhi interacts with his congregation during a Sunday service. He denies that Sandra's trafficker was an assistant pastor at his church. "You know pastors do certain things," he said. "I don't call them pastors, I call them herbalists or native doctors in suits who would do such."

The betrayal that stretched across two continents is now even closer to Sandra.

"Even my own father he said I am not his daughter," she says.

The trafficking is not Nigeria's problem to solve alone, says Okah-Donli, but it is Nigeria's tragedy.

"It's our young boys and girls who are trafficked. Many are not making it back alive and the ones that do are battered and bruised."