They call it the "cage" or the "jail". Tucked away in the small Bulgarian town of Sredets is a crumbling outbuilding on the edge of a compound where border forces are stationed.
The floor is often covered with litter, there are no proper beds and at one side are metal bars.
The squalid room is no ordinary prison. People who say they've been held inside deny they're criminals.
Instead, they are asylum seekers who claim they were detained in a makeshift jail by Bulgarian border forces before being forced back to Turkey.
"When they caught us, they beat the hell out of us... they stripped us naked... shaved our brows with a Gillette razor," said Nackman, a 34-year-old refugee who claims he was detained for several hours in the cage.
"Then there was this wooden room that could only fit 20 people, but they forced 60 people into it; even an animal couldn't survive in that wooden room. It has the worst odour ever, so if you need to use the toilet, you'll just go to the side. They won't look at you with human eyes."
As part of joint investigation by Sky News, Lighthouse Reports, Der Spiegel, Le Monde, Monitor, Domani, SRF and RFE/RL Bulgaria, we filmed the outbuilding five times in October and November 2022.
Every time there were people inside. On one occasion, a Bulgarian border force officer is recorded opening the door and leading the people out.
We showed the video of the outbuilding to refugees near the Turkish-Bulgarian border. Kenan, 31, instantly recognised it.
"We slept in this jail for three days. They insulted us. When we asked them for bread, they pushed us back, cursed us and I didn't know what they said. They hit us," he recalled.
'They unleashed the dogs'
Another man said he was held there before being pushed back over the border to Turkey.
"After they threw me in the container, they unleashed the dogs. It was me and a young child. He was 12 years old. He was bitten by the dogs and so was I," he said.
Claims of abuse at the border by Bulgarian authorities are common. A Syrian refugee named Shaddi said on one occasion he was caught by authorities trying to cross through Bulgaria to Serbia.
"They took off our clothing and they put us in very cold water, maybe 10 minutes. And they threw sticks... and they shoot us by plastic shoot revolver. It's plastic shot," he said. "It's unhuman I think... because we are human. We are not animals. We are not terrorists."
Read more: Refugee shot near Bulgarian border
As part of the investigation, reports of illegal pushbacks and abuse were recorded at several EU borders. In Hungary, refugees reported being held for hours tied up outside or in containers.
Speaking anonymously, one man said he was badly beaten by Hungarian police and detained for two days. Similar reports have been told to aid workers at Doctor Without Borders.
"What we started hearing more and more is that people were kept inside such containers up to 12 hours just before being forcibly returned to Serbia. Often kept up to 60 people in such a contained space, in a standing position or in physical stressing positions," said Alessandro Mangione from the medical NGO.
Hungary 'utterly rejects allegations'
He said aid workers regularly treat refugees for beating injuries caused by officials.
In response to the allegations, the Hungarian authorities issued a statement saying its officials carry out their duties lawfully, professionally, humanely and proportionately.
"The government of Hungary utterly rejects allegations which are once more seeking to discredit personnel on duty at the border. Hungarian police officers and soldiers are protecting the EU's Schengen borders lawfully and in compliance with EU and Hungarian regulations," it added.
The Bulgarian government denied any wrongdoing by border officials and said it followed international and domestic laws. It said officials were facing increasing attacks from people trying to cross illegally into Bulgaria.
The European Commission said it took all allegations of wrongdoing at the European external borders seriously, that violence and loss of life was unacceptable, and it expected national authorities to investigate any allegations and to follow up swiftly and effectively.
But the revelations are concerning; people seeking protection are reporting abuse and detention in clandestine prisons - alleged black sites in Europe.