When Mohammed worked in his owner's field in Libya, bent over for hours pulling tomatoes from the soil, he would think often of the days when he was a free man.
He had lived a modern life in Zinder, Niger's second-largest city. He grew up in a good family and learned in school to speak two foreign languages — English and Arabic. "I kept thinking, I'm a human being, just like him," he says, comparing himself to a man in Libya he says enslaved him.
Mohammed fled Niger when he was just 17 years old, when extremist Boko Haram militants came to his area. He paid smugglers to take him across North Africa and to Europe, but when they reached Libya, he and other migrants were detained by a Libyan militia.
"My mistake was that I came to this country [Libya]," he says.
There began a three-year ordeal that saw him thrown into prison, held for ransom, and bought and sold into bonded labor or debt slavery.
Mohammed is now 20. He looks haunted by his time in Libya, and asks not to use his last name, afraid that the militia that caught him in Libya might find him in southern Tunisia, where he now lives.
Testimonies like Mohammed's, recorded by NPR, suggest a pattern of slave-trade abuses of migrants detained in Libya. The conflict-riven country is a final stopover for many people from across Africa and parts of Asia taking long, risky journeys for refuge in Italy or onward in Europe.
Italy has sought to reduce the number of arrivals by financing and training Libyan authorities to stop the migration, even as rights experts decry inhumane conditions for asylum-seekers in the North African country.
On Wednesday, the United Nations human rights office reported that migrants in Libya have been bought and sold at "open slave markets."
Now, some migrants who escaped are speaking out and detailing harrowing experiences.
NPR met Mohammed through Mongi Slim, the southern Tunisia director for the Red Crescent aid group, which is sheltering hundreds of migrants who arrive from neighboring Libya.
Slim says Mohammed's experience of slavery in Libya — in which Mohammed claims he was sold to a man who used him for farm work, before he finally escaped — has become commonplace.
A pharmacist by training, Slim has also been a volunteer with the Red Crescent for more than 20 years. He's long been familiar with the mistreatment of migrants by human traffickers, smugglers and militias in Libya.
Libya still reels from its civil war in 2011. Rival governments in the east and the west wrestle for power, and militias run the streets.
But since mid-2016, Slim says, more and more migrants have arrived bringing testimony of being traded as commodities and being sold into slavery. "At first, I was a bit skeptical. If just one person comes with this story, of course you are not sure if it's truth. But then when so many people came and told us the same thing, it became more clear," Slim says.
Late last year, a CNN investigation revealed footage of what appears to be African migrants being auctioned off. In the video, an auctioneer sells men he describes as "big strong boys" to the highest bidder.
The footage caused international outrage. In December, the United Nations Security Council condemned the reported acts of slavery in Libya as "heinous abuses" and called "for those responsible to be held to account."
In southern Tunisia, Slim says he started hearing accounts of slavery long before they came to light in international media.
Most of the men and women at the Red Crescent shelter come from African countries. Slim introduced NPR to migrants from Gambia, Nigeria and Senegal. They sleep on foam mattresses on a concrete floor, several to a room.
Five young men want to discuss their experiences. They didn't witness the secret slave auctions reported by CNN, although they say they have heard of those. Instead, they describe Libya's detention centers — a network of prisons, some of them sanctioned by the state as part of an effort to combat illegal immigration — as being the very places where migrants are "bought" and "sold."
Boubaker Nassou, a 30-year-old from Gambia, describes the detention center where he was held as a "slave market" and as the "prison where they sell people."
Like Mohammed, from Niger, the smugglers' car that Nassou rode in was detained by a militia once he crossed into Libya. The migrants were bundled into another vehicle, and he says they were taken "straight to the prison where they sell people."
It was a large, dank room, crammed full of African men and women. Nassou says he barely had space to sit or lie on the concrete floor. It was hot and no one was allowed to shower; the air was putrid.
And all the while, Nassou says, Libyans would come to haggle with the guards. What they were haggling over was the price for prisoners. He says a "business" has developed of buying migrants out of one detention center and selling them to another for profit.
"This man would come and say 'I need one person,' and they say, 'This one is 400 Libyan dinar.' 'This one is 500.' 'This one is for 300' and 'this one is for 200.' They sell you and buy you like that," Nassou said. (In U.S. dollars, that's a range of about $150 to $350.)
Shortly after he was detained and thrown in detention, in 2016, Nassou says he was bought by a Libyan man for 500 Libyan dinars — the equivalent of $350.
He doesn't know the man's name; in the prison, he says he was taught to just refer to every Libyan he met as mudir, Arabic for "boss."
He says the man moved him to another detention center, and sold him to the guards for three times the price.
These accounts are consistent with some of the reporting by humanitarian and rights organizations on the mistreatment of migrants in Libyan detention centers.
A report by the U.N.'s International Organization for Migration in 2017 also documented what it called "slave market conditions" for migrants in Libya. The report included testimony similar to Nassou's, of migrants being bought and sold for profit between militias and detention centers.
An Amnesty International report published in December estimates there are over 20,000 refugees and migrants currently being held in official detention centers that are — in theory — run by the Libyan Interior Ministry's Department of Combating Illegal Migration.
"There are 33 official detention centers under the DCIM," said Marwa Mohamed, a researcher for the Amnesty International report, using the department's acronym. "But the DCIM has nominal control over them. They are in fact run by militias in certain areas and there's very little oversight."
Thousands more migrants are being held in places run by criminal gangs, separate from the government based in the capital Tripoli. But, Mohamed says, human rights abuses are happening in both types of places.
Libyan law has few provisions for asylum-seekers or protections for victims of human trafficking. It criminalizes irregular entry and punishes undocumented immigrants with fines or jail while they wait to be deported.
The Amnesty International report, which is based on interviews with dozens of migrants, says that prison guards extort the inmates and demand ransom from their relatives. In some cases, guards will torture a prisoner so that their loved one can hear their screams on the phone.
When they can't extract payment, the report says, there are cases where the prisoners are sold into bonded labor, a form of modern slavery. Someone will come and pay the prison to free an inmate, but from that moment the prisoner is beholden to the person who paid for their release. Often they are forced to work off the debt.
"Everyone who gets out of these detention centers has either somehow miraculously escaped, has been sold, or has to pay a fee — a large ransom — in order to get out. These are your options," the researcher Mohamed says.
An official in the Tripoli government declined NPR's request to comment on the treatment of migrants in Libya.
Libyan authorities have called for investigations into the sale of migrants. And in recent months the immigration department that oversees the detention centers has taken steps to close down centers where abuses happen, according to Amnesty International.
Nonetheless, migrants say the abuse — and selling — of migrants in Libya remains prevalent.
In its report, Amnesty also accused Europe of continuing to "outsource border control to Libya" despite the horrors suffered by migrants there.
Several migrants in the Tunisian shelter describe experiences of being sold into bonded labor in Libya.
Some of the accounts are cruelly bizarre — a young man says he was dressed up as a woman by his Libyan owner who then tried to sell him to a sex trafficker.
Nassou says in 2016 he was sold out of a detention center in the northwestern coastal town of Zawiya to a Libyan who needed a "painter man." The man who bought him took him back to his house and made him paint all the rooms, the Gambian migrant says.
Nassou says his buyer seemed to take pity on him and let him go after his work was done.
Others are not so fortunate. Mohammed, from Niger, says the Libyan who bought him from a prison was in fact paying his ransom that had been set for him by guards. "He paid the amount of money, which means, he had bought a slave," he said.
But Mohammed says, there was no contract, no promise of how long he'd have to work before he could have his freedom.
"If you're not Libyan they don't think you're human. You're an animal in their eyes," he says, rage flashing across his face.
Sometimes the farmer would give him food, sometimes he wouldn't, says Mohammed. The way he was treated depended a lot on the farmer's mood. "You know, sometimes, he would [be] happy and sometimes he would be sad," Mohammed says. "When he was happy, he would share his happiness with me."
"And when he was down..." his voice trails off. He later describes being beaten, and given only scraps of food to eat.
One time, he says, he built up the courage to ask the farmer for payment for his work. "I asked how he was OK with not giving me what was rightfully mine," he says. The farmer's answer was to pull out a gun and shoot, narrowly missing him.
Mohammed slept in the dirt outside the farmer's home. The farm was set in a remote scrubland far from any other settlements. But at night he could make out headlights twinkling faintly in the distance.
"They were far away but you could see them," he says. One night after seven months of captivity, Mohammed says he had enough. "I got fed up with the hunger and the exhaustion and the beatings and the sickness. I didn't have a watch, but it must have been around 2 to 2:30 in the morning. Everyone was asleep. I ran." He sprinted for three hours to reach the road he'd seen from so far away and hitched a lift to the capital Tripoli.
Eventually Mohammed managed to earn enough money from odd jobs to pay smugglers to escape the country. He boarded a small, overcrowded boat that broke down in the Mediterranean Sea. He was rescued by the Libyan coast guard, which brought him to Slim from the Red Crescent.
Slim says he tries to help the migrants as he can. He found Mohammed a job as an olive picker in a grove not far from Zarzis, a town on Tunisia's southeastern coast.
He's still extremely poor. He sleeps on a foam mattress in a tiny, dark and empty concrete room. But in comparison to what he's been through in Libya, he says, abject poverty is a small problem: "Here the work is little, and the money is bad. But at least there's safety, and that's what I want — to live my life in a safe place."
Ruth Sherlock and Lama al-Arian reported in Zarzis, Tunisia.