The police in South Africa said on Wednesday that they had arrested a man who confessed to starting a fire in a derelict Johannesburg building last August that killed dozens of people, some of whom leaped to their deaths or were trapped behind locked security gates.

The 29-year-old man was arrested on Tuesday on 76 counts of murder and 120 counts of attempted murder, said Col. Dimakatso Nevhuhulwi, a spokeswoman for the police in Gauteng Province, which includes Johannesburg. The man’s name has not been released.

Officials originally said that the fire, which tore through an overcrowded four-story building in the early hours of Aug. 31, had killed 77 people. But a commission investigating the disaster was later told that the number was in doubt because of how badly some of the bodies were burned.

The fire focused international attention on hundreds of rundown, illegally occupied buildings in Johannesburg like the one that burned, where poor families who cannot afford more secure housing have settled. These urban squatter camps are known as “hijacked” buildings because they are often taken over by gangs that deal drugs and extort residents for rent payments.

The 29-year-old man was arrested after testifying on Tuesday before the commission investigating the fire. The commission’s hearings have been live-streamed, but the man testified behind closed doors out of fear that gang leaders who had been operating out of the building would have him killed, the commission’s lead attorney said.

“His life may be in peril if made public,” the attorney, Ishmael Semenya, said during a public portion of the hearing.

The police did not provide details about the man’s reported confession. According to South African news outlets, he said in his testimony on Tuesday that he had strangled a man, poured gasoline over the body and set fire to it, and that the fire had spread.

Those killed in the blaze included at least a dozen children — two of them toddlers named Memory — migrants from other African countries and people holding down jobs as teachers and technicians. Neighbors and residents said the building also housed criminal gangs that sold drugs on the sidewalk and robbed passers-by.

The government owned the building and many others like it. Going block by block and searching records, New York Times reporters last year identified at least 127 similar buildings, neglected death traps, in the center of Johannesburg.

President Cyril Ramaphosa has said that the fire was a “wake-up call” for South Africa, where the cost of living puts housing out of reach for many, and where city governments look the other way as people occupy garbage-strewn buildings without water or electricity.

The commission investigating the fire halted its work last year after the building that housed it was itself deemed to be a fire hazard. Its hearings resumed this month.