(CNN) At the heart of the deadliest migrant route in the world, a small number of rescue boats brave the choppy waters of the winter Mediterranean. They are picking up stranded migrants attempting to cross from Africa to Europe, with many telling horror stories of imprisonment, brutality and exploitation as they had passed through Libya.

For 11 days in December, CNN was the only media on a boat operated by Proactiva Open Arms, a Spanish NGO that rescues migrants who run into difficulty trying to cross the Aegean and Mediterranean Sea. During its mission, Proactiva picked up 695 migrants from 27 countries, who had all set sail from Libya -- over one third of all the migrants who reached Italy in December.

Among them was Nigerian Celestine Ike. The 27-year-old claims he spent nine months stuck in Libya, where he says he had been captured and imprisoned for almost four months, until a friend paid his ransom of 4,000 Libyan dinars ($2,930).

"A lot of blacks are suffering there," he said. "They are using black men as slaves."

'Killed like animals'

Ike refers to his captors as "Asma boys," a term that many sub-Saharan migrants use to describe those in the business of holding migrants for ransom. Several migrants said that the Asma boys can belong to either criminal gangs, militias or the Libyan police.

Ike said that while he was imprisoned he had been repeatedly shot by his captor. With a damaged finger on his left hand, which he says was smashed with a hammer, he pointed at a bullet wound on his foot. He said that in prison he saw many people dying, and most of them were black.

"You stay there and you see your fellow human beings being killed like animals," he said.

After his ransom was paid, he says he was pushed at gunpoint onto a dinghy carrying 113 more migrants, from which he was rescued by Proactiva on December 15. The craft had left the western Libyan city of Zuwarah that day. After nine hours at sea, it was found drifting aimlessly, having run out of fuel.

All the rescues carried out by the Open Arms were of boats that had departed from one of three cities -- Sabratha, Zuwarah, and Zawiya -- some of the country's biggest trafficking hotspots, according to Amnesty International.

Detention and physical violence

Many of those rescued told CNN stories similar to Ike's. They painted a picture of Libya as a place where sub-Saharan migrants journeying to Europe are routinely captured, robbed and exploited by locals.

"In Libya, you cannot do anything," said Ike. "If you're walking in the street, you're in trouble. If you work, Libyan men won't pay you. If you ask for your money, they'll threaten you.

"The father has guns, the mother has guns, the children have guns -- they all have ammunition to threaten blacks."

Mohammad Guray Farax Quule, a 23-year-old from Somalia, said he was imprisoned three times in Kufra and Bani Walid, and was extorted for a total of more than $8,000. He says he was once forced to be present as 10 of his captors raped one woman.

Diallo Alhassane, 17, from Guinea, said that a Libyan farmer, for whom he was forced to work for four months, once threw into his cell the dead body of a Sudanese migrant who had tried to escape. Alhassane too showed a bullet wound on his foot.

Rescued migrants wait to board the Open Arms.

survey conducted by the IOM found that underage migrants on the Mediterranean route are the most exposed to human trafficking, with a vast majority of them having experienced detention and physical violence in Libya.

Evacuating detention centers

While the number of migrants arriving in Italy from Africa in 2017 is down a third from 2016 according to Interior Ministry's data, fewer rescues are being carried out by European Union military vessels in the area adjacent to Libyan state waters. Proactiva says that means the burden of rescuing migrants increasingly falls on non-profit human rights organizations like theirs.

The fall in rescue missions by European military comes as the EU pursues a strategy of supporting the Libyan Coast Guard "to enhance their capacity to effectively manage the country's borders." The EU has allocated €46 million ($54,6 m) to this program, and Italy has provided training and equipment to the Libyan Coast Guard.

But this strategy has been criticized by humanitarian organizations such as Amnesty International , Doctors Without Borders and the United Nations, because migrants intercepted by Libyan authorities are returned to Libyan detention centers.

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In September, the UN High Commissioner for Human Rights, Zeid Ra'ad Al Hussein, said that "returning people to detention centers where they are held arbitrarily, and face torture, rape and other serious human rights violations, is a clear breach of the international law."

CNN reached out directly to the EU and Libyan authorities for comment, but hasn't received a reply.

On December 8, a UN-run plan of mass evacuation of Libya's detention centers kicked off with the voluntary repatriation of 504 migrants to Niger.

The EU's High Representative for Foreign Affairs Federica Mogherini said that the EU's objective "is to close the detention centers and make sure that they do not exist anymore in the form they exist today."

After a meeting with representatives of the African Union last week, she defended the European aid to the Libyan military, saying that "this money, that the European Union dedicated to assisting Libya itself, has a component that is related to border management, both at land and at sea, and has a component that is, on the other side, addressing the social needs, the development needs, and the needs of the population of Libya."

Libya's Interior Minister, Al-Aref Al-Khouja, said he had launched an investigation into human rights abuses against migrants' detention. "We will not be complacent with those who violate these rules and principles of humanitarian treatment, whether it's with our own citizens, or any foreigners, and this is a global humanitarian demand which people will not accept anything less than," he said in a news conference.

'We slept on the floor and they raped the women'

Activists say that the proliferation of illegal prisons in Libya, where armed groups hold migrants for ransom, means that no clear figure is available on how many people are currently being held in detention centers.

But for migrants, the distinction between an official detention center and an illegal prison is blurred.

Suzanne Ebudou, a 25-year-old from Cameroon, says she was arrested and held for ransom after entering Libya nine months ago. She said that after paying 1,000 dinars ($735), she was taken to what she was told was a refugee camp. "But I don't believe it was a real refugee camp, because they mistreated people. We slept on the floor and they raped the women," she said.

She says that she and nine other Cameroonian women would sleep together in a locked room, to protect themselves from rape. But then, she says, the guards started to drug their meals.

"Immediately after dinner you fall asleep, then the guard comes to get you and you wake up the next morning in his room, and when he's done with you, he brings you back to the prison," she said.

The end of the nightmare

Those who do make it out of the detention centers, and take to the sea on packed, unseaworthy boats, rely on rescues by European vessels for their survival. Two Egyptian teenagers, traveling with their family said they had used a phone app called "Marine Traffic" to check whether the Open Arms was in the area when their smugglers pushed their boat into the sea.

That thought weighs heavy on the mind of Proactiva's volunteers, like Jorge Pacheco, who piloted one of the Open Arms speedboats during search operations that could last up to three hours, often at night time. The Argentine national carried out 11 rescues over one week. "It is an exhausting job," said Pacheco, who in summer skippers luxury yachts. "But if we weren't here, these people would die."

Left, Proactiva's Head Of Mission Dr. Guillermo Cañardo, center, Mohammad Guray Farax Quule, and right, Jorge Pacheco.

For the migrants, rescue means the end of their nightmare, and hope for the future. As Ike climbed aboard the rescue boat that would grant him a safe passage to Italy he said, "Today I am so excited, it is my happiest day, because today I am a free man.

"I believe that if God helped me to make it from Nigeria to Libya, and to survive in Libya. God will also help me in Europe."