A day earlier the 24-year-old pharmacy assistant was detained by soldiers from South Sudan's majority Dinka tribe, while taking medicine to his sick father outside the southwestern town of Yei.
Ethnic killings rampant in once-placid South Sudan town
Isaac waited as soldiers came to take away his cellmates, one by one, fearing the worst as the uniformed men returned alone and spattered in blood.
Uganda, one of the world's poorest countries, currently hosts 530,000 South Sudanese refugees, 330,000 of whom fled fighting in the world's newest country this year alone.
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Searching his bag, the soldiers discovered the medicine and knew by his language that he was a member of the Kakwa ethnic group.
They accused him of trying to supply medicine to anti-government rebels -- many of them Kakwa -- hiding in the bush and took him to the small room on a military base that was to be his prison.
Four others shared the cell overnight. A man named James was the first to be called out.
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"They said he was going to fetch water. But we waited all evening, that guy was not back until now," said Isaac, who spoke to AFP in a refugee camp in northern Uganda and did not want to give his last name.
Later, when the soldier returned, "his chest was bloodied even the legs was full of blood." A second man was called out.
Fearing for their lives Isaac and his remaining cellmate began to pray and cry out.
The Sudan People's Liberation Army (SPLA) is dominated by Dinkas but includes other tribes from the Equatoria region where Yei is situated. Five soldiers responded to their shouts.
Soldiers caned by comrades
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Hearing that two Kakwa prisoners had disappeared, the Equatorian soldiers went to their Dinka commander. "He said that's not their concern," said Isaac.
From the window of his cell Isaac could see what happened next.
At the order of the commander, the five Equatorian soldiers were made to stand in the sun and each was caned 50 times.
Hours later those soldiers turned their guns on their officers and demanded the prisoners be released.
The cell door was opened and Isaac fled. Days later he was on the road with his family walking to Uganda.
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Ethnic violence has characterised South Sudan's political crisis since conflict erupted in 2013, and refugees fleeing Yei have described a disturbing pattern of targeted killings in the town that was once a haven of peace in the war-torn country.
The conflict initially pitted Dinka and Nuer supporters of President Salva Kiir and his former vice-president Riek Machar against each other.
However observers say it has metastasised with other tribes joining one side or the other, often with the hope of getting an upper hand in local conflicts over land and other issues.
In Yei, violence has surged since the collapse of an August 2015 peace deal between Kiir and Machar led to an outbreak of violence in the capital Juba, sending thousands fleeing to Uganda.
Dead piled into lorries
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Refugees told AFP how SPLA forces went door to door, killing civilians from tribes they accuse of supporting the rebels.
"At night they don't shoot guns because people will realise there's a problem... they will call you to come out and use knives," said Isaac, who said many of his former school mates fight with the rebels.
He said he once saw the dead piled into a convoy of three lorries and taken away.
"You see blood on the vehicle, blood flowing, that's when you realise there are dead bodies going. They take them to the barracks and they have that vehicle for making holes then they dump them there."
However both sides have been accused of atrocities.
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"Now only Equatorians are being killed but when the rebels enter the town that's when you find that the Dinka are being shot," said Isaac.
His story echoes those told by over 20 refugees interviewed by AFP.
Government 'targeting civilians'
Irene, from the Lulobo ethnic group, fled Mambe in Yei River State when Dinka men raided her house and called for her husband by name.
Irene said she knew they were Dinka from the distinctive traditional facial scars.
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"The person who shot him just knocked at the door and called by name. When he heard that he pushed me under the bed with the children," said the 28-year-old.
The soldiers burst in, shone a torch around and shot her husband.
Irene fled with her children but while walking through the bush to Uganda she and the group she was with encountered another group of armed Dinka men.
"Two women were trapped and raped by the men. The men took two infants -- about a year and about 18 months -- and used them as if they were sticks to beat the women.
"The children survived but I think they were damaged inside," said Irene. She did not know why she was spared.
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Hundreds of thousands of South Sudanese refugees now sheltering in neighbouring countries have fled similar horrors around the country.
jpegMpeg4-1280x720Adama Dieng, the UN's Special Advisor on the Prevention of Genocide, recently warned of the "potential for genocide".
Nasir Abel Fernandes, a senior UN refugee agency (UNHCR) official working in northern Uganda agreed.
"It's a pattern that everybody tells us who comes across the border, that they feel they're being killed because of their ethnic dimensions," Fernandes said.
Warnings have grown louder that fighting could intensify.
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"We have credible information that the South Sudanese government is currently targeting civilians in Central Equatoria and preparing for large scale attacks in the coming days or weeks," Keith Harper, the US representative at the UN Human Rights Council, said in Geneva Wednesday.
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