After crawling into her cell, Lee Young-joo was ordered to sit cross-legged with her hands on her knees.
She was not allowed to move for up to 12 hours a day.
A slight shuffle or a hushed whisper to her cell mates would be harshly punished.
She had limited access to water and was given only a few ground corn husks to eat.
"I felt like an animal, not a human," she said.
She told the BBC she spent hours being interrogated for doing something many of us take for granted - leaving her country. She was trying to escape North Korea in 2007 and was caught in China and sent back.
She spent three months at the Onsong Detention Centre in North Korea near the Chinese border, waiting to be sentenced.
As she sat in her cell she listened for the "clack clack clack" of the metal tips of the guard's boots as he patrolled outside. Backwards and forwards he went. As the sound went further away, Young-joo took a chance and whispered to one of her cell mates.
"We would talk about plans for another defection, plans to meet with brokers, these were secretive talks."
Prison was supposed to deter people from escaping North Korea - it clearly didn't work on Young-joo or on her cell mates. Most were waiting to be sentenced for trying to leave the country.
But Young-joo's plans had been overheard.
"The guard would ask me to come to the cell bars and put out my hands, then he started beating my hands with a key ring until it got all bloated and blue. I didn't want to cry out of pride. These guards consider those of us who tried to leave North Korea as traitors.
"You could hear others getting beaten because of the cells sharing this corridor. I was in cell three but I could hear beatings from cell 10."
A system of suppression
Young-joo is one of more than 200 people who have contributed to a detailed investigation by Korea Future into violations of international law within North Korea's prison system.
The non-profit organisation has identified 597 perpetrators linked to 5,181 human rights violations committed against 785 detainees in 148 penal facilities.
The evidence has been gathered and put in a database in the hope that one day those responsible can be brought to justice.
North Korea has always denied allegations of human rights abuses. The BBC has attempted to contact a representative from North Korea to respond to this investigation but received no reply.
The group has also created a 3D model of the Onsong detention centre to allow people to see the conditions for themselves.
Korea Future co-director in Seoul, Suyeon Yoo told the BBC that the prison system and the violence within it was being used to "suppress a population of 25 million people".
"In every interview we conduct, we witness how this has impacted human lives. One interviewee cried as she recounted witnessing the killing of a newborn baby."
Multiple allegations of abuses
North Korea is currently more isolated from the world than it has ever been.
The country has been ruled by the Kim family for three generations, and its citizens are required to show complete devotion to the family and its current leader, Kim Jong-un.
The Covid pandemic has brought even stricter controls both within the country and at the border.
Tougher prison sentences have been imposed on those who try to get a glimpse of the outside world - including those who watch foreign dramas or films.
The pattern of violence within the system is repeated in testimony after testimony and in prison after prison.
There are multiple allegations of rape and other forms of sexual assault. Survivors also told the organisation they were forced to undergo abortions.
In one case in North Hamgyong Provincial Holding Centre, an interviewee witnessed a fellow detainee being forced to have an abortion while eight months pregnant. She claims the baby survived, but was drowned in a basin of water.
There are five cases where witnesses describe executions.
One step closer to justice
Young-joo was eventually sentenced to three-and-a-half years in prison.
"I was worried whether I would still be alive by the time I finished my sentence," she said. "When you go into these places, you have to give up being human to endure and survive," she says.
Saerom was also in Onsong Detention Centre in 2007, but she recalled that the beatings at the State Security prisons were worse.
"They beat your thigh with a wooden stick. You walk in but you crawl out. I couldn't look at other people being beaten and if I turned my head away they would make me look at it. They kill your spirit."
"If there is a way, I want them to be punished," Saerom told us as she recounted the recurring nightmare of her time in prison. She said she now takes pleasure in every moment of happiness in her new life in South Korea.
Prosecuting these cases will be difficult, however this investigation has had input from experts from the International Criminal Court. The evidence will also be admissible in court and it is being made freely available.
Saerom and Young-joo both told us that they hope this report will bring them a step closer to the justice they crave.