An investigation into the cause of a fire in Johannesburg that killed 77 people began on Thursday, focusing attention on gangs that acquire abandoned buildings in the city center and unlawfully rent them out.

The fire broke out on Aug. 31 in a rundown building filled with largely foreign migrants, becoming one of the greatest disasters in recent memory in South Africa’s economic powerhouse. Many of the victims were severely burned.

Residents said at the time of the disaster that the building had been taken over by criminal syndicates who charge fees to occupants, exploiting them but also offering more affordable housing to those who might otherwise be left homeless.

“When a building has been hijacked it becomes a fire time bomb,” Acting Chief of Emergency Management Services for the City of Johannesburg Rapulane Monageng told the inquiry.

He listed a litany of fire hazards identified so far: blocked emergency escape routes, stair wells converted into makeshift rooms and wood brought in from outside to partition rooms to create makeshift wooden shacks on the ground floor, where some 200 people were crammed together, he said.

A lack of fire extinguishers and the diverting of water from fire suppression equipment for domestic use were also evident.

Residents of such buildings rely on illegal electricity connections, gas burners and – when the country’s rolling power blackouts hit – candles, all of which increase the risk of fires.

Johannesburg is one of the world’s most unequal cities with widespread poverty, joblessness and a longstanding housing crisis. It has about 15,000 homeless people, according to the government of Gauteng, the province that includes the city.

Johannesburg city authorities had leased the gutted building to a charity providing shelter for women in 2016. But the charity ran out of money and stopped operating, residents said.