As Tom Mandala leaned out of the fifth-floor window of his burning apartment building in Johannesburg early Thursday, it felt as if the only decision left to make was how to die.

He could turn around and dash for the stairs, but he would surely be overcome by the thick smoke and scorching flames, he figured. Or he could leap out of the window and end up splattered on the sidewalk below.

The second option, he thought, would be the best way to ensure that his family back in Malawi would be able to recover his body. So, after about five minutes of agonizing deliberation, Mr. Mandala, 26, jumped.

“I was thinking nothing,” he said of the moment when he soared through the air.

Landing on his feet sent a rush of pain into his lower legs so sharp that tears began to flow, he said. His right ankle was broken, and his left leg badly injured. But he was alive.

Mr. Mandala was among the lucky survivors of a fire that killed at least 74 people and injured dozens of others on Thursday, one of the deadliest residential blazes in South Africa’s history. The derelict building in downtown Johannesburg had been taken over by informal landlords to become a sprawling settlement that was a port of last resort for hundreds of struggling South Africans and immigrants searching for a break in one of Africa’s most advanced economies.

As investigators sorted through the ashen rubble on Friday, more details emerged about the chaotic, horrifying conditions inside a building that city officials said was so unsafe that it should not have been occupied in the first place. The building had been “hijacked” by criminals who extorted money from the working poor who could not afford formal housing, officials have said.

Interviews with survivors who lived in the building revealed that even though the city-owned property was not a formal apartment, it operated like one, with residents paying monthly rent to people they called landlords.

On Friday morning, Abdul Manyungwa, a local business owner and Malawi native who has lived in South Africa for 11 years, was at the scene gathering contact information from the building’s surviving residents to try to help arrange shelter for them. Most were from Malawi, he said, a sliver of a southern African nation that has among the highest rates of poverty in the world, according to the World Bank. Others were from South Africa, and a few from Zimbabwe and Tanzania.

Although many of the residents were immigrants, the people they were paying their rent to appeared to be South African. Several residents described their landlords as men who spoke isiZulu, the mother language of South Africa’s Zulu people. They paid rent ranging from 600 rand ($32) to 1,800 rand per month, depending on the size of their families, and that included electricity and water provided through illegal connections.

Residents and city officials described a building that was a firetrap. There were no fire exits or sprinklers. Rooms were subdivided with cardboard and sheets, and some residents lived in dozens of tin shacks constructed inside the building in an open space on the ground floor.

The authorities have said that many of the fire’s victims were trapped behind a locked gate, and Mr. Mandala said there was such a gate at the bottom of the stairwell leading to the ground floor. He did not have a key, so every time he wanted to leave the building, he had to wait for someone with a key to open it.

Despite the drab conditions, he said, the building had been a blessing for him.

He moved to South Africa a year ago after his efforts to find work as a police officer and teacher in Malawi failed. He had heard of other Malawians coming to South Africa and earning enough to build nice homes, so he figured he could follow the same path.

But when he arrived, he found it just as much of a struggle to earn a living in South Africa, an economic powerhouse on the continent. He worked selling cellphone accessories, a job that paid 2,000 rand ($107) a month. He initially lived in a building where he had his own bed and paid rent of 1,500 rand a month.

Mr. Mandala said he had found out about the building where Thursday’s fire broke out from a co-worker and, three months ago, moved into a room there that he shared with four other Malawians. The five of them shared two beds, and he paid 600 rand a month.

With the reduced rent, life was still hard but was much more comfortable, he said.

Until Thursday morning, when a roommate lying next to him jolted him awake. When he opened the door to their apartment, he was overcome by smoke in the hallway, he said. So he broke open the window and peered out.

Only four of them were home at the time, Mr. Mandala said. He encouraged his roommates to jump out the window as well. One of them followed, and he, too, survived. The two who did not, Mr. Mandala said, remain missing.