It was time, the witness said, to tell the whole truth about the terrible things he had done.

Shortly before testifying this week at an inquest into one of South Africa’s deadliest residential fires, he pulled an investigator aside and said he had to change his story. He was the one, he said, who started the fire on August 31 that devastated a five-story building in downtown Johannesburg.

As he spoke, 30-year-old Sithembiso Mdlalose, sobbing, told a room full of stunned listeners that he strangled a man on the ground floor of the crowded, abandoned apartment building and set the body on fire.

After this confession at the inquest, Mr Mdlalose got arrested by police, who are conducting a parallel criminal investigation into the fire, and charged with 76 counts of murder. But as he appeared in criminal court for the first time on Thursday, there was still much confusion and mystery surrounding this shocking twist in a tragedy that has drawn international attention to the terrible living conditions of thousands of people indoors in one of Africa’s richest cities.

While Mr Mdlalose’s confession was inadmissible in the investigation, prosecutors said his confession would strengthen the criminal investigation.

Mr Mdlalose, who said he lived in the building, seemed to appear out of the blue to lawyers and activists working with some of the building’s former residents, many of whom mostly said they did not know him. He was one of many witnesses scheduled to speak at the inquest, which began in October.

Before appearing at the inquest, he gave police a handwritten statement that went far beyond what he ultimately admitted. In his statement, obtained by The New York Times, Mr. Mdlalose admitted that he was one of the many criminals who sold drugs in the building, but suggested that others had killed the original victim and set him on fire.

Mr Mdlalose gave this statement to the police about a month ago. A spokeswoman for the South African Police Service declined to comment when asked why he had not been arrested sooner.

Some victims’ advocates fear Mr Mdlalose’s bombshell statement this week could obscure the wider failings that led to the fire and that the inquiry is set to uncover. Testimony so far has shown that the illegally occupied building, which is owned by the city, was rife with fire hazards that were not addressed by officials, said Nigel Branken, an immigrant rights activist who works with survivors of the fire.

“My concern is that the city will say, ‘Great, we have someone,’ and then they can get out of their obligations,” Branken said.

“The city is responsible for more deaths than the man who started the fire,” he added.

Mlimandlela Ndamase, a spokeswoman for the mayor of Johannesburg, urged caution before passing judgment.

“Let us wait until the investigation is completed and then we will understand what responsibility or liability will be imposed on whom,” he said.

This week, guilt and fear drove Mr. Mdlalose to confess that he set the fire, according to Nomzamo Zondo, executive director of the Socio-Economic Rights Institute, who was present at the inquest hearing. Another person who was present but was not authorized to comment said Mr Mdlalose pulled aside one of the people in charge of giving evidence and said he wanted to tell the truth.

Mr Mdlalose, whose age police said earlier this week was 29, spoke to investigators behind closed doors, fearing that the criminals operating in the building would kill him, but lawyers and activists who were in the room , told his story afterwards.

He testified that he was working for a drug dealer when he was called to beat up a man who had gotten into an argument with the dealer, the observers said. Mr Mdlalose, who said he was high on methamphetamine at the time, said he found the man already tied up and with a bag over his head in a room in the building.

He beat the man until he was unconscious and bloody, and it was only when he took off the bag that he recognized the man, he said. Panicking that he might be identified if the man survived, he told investigators he strangled him with his bare hands, then doused the body with gasoline and set fire to the room to hide his crime.

In his previous written statement to police, Mr Mdlalose admitted to beating the man along with others. In that statement, he said the man, who was left slumped in a chair, died from his injuries. He and his accomplices poured gasoline on the man’s body and spread it around the room, but left without setting it on fire, the statement said.

At the public inquiry, Mr Mdlalose said he decided to come clean when someone who saw him carrying the petrol into the building persuaded him to vomit. This witness testified at the inquest the day before.

As Mr Mdlalose stood in court as a defendant on Thursday, wearing a faded khaki parka with lint in his hair, he barely spoke a whisper as a judge explained the lengthy prison sentence he now faces for multiple counts of murder, attempted murder and arson.

Phindi Mjonondwane, a spokeswoman for the public prosecutor’s office, said prosecutors were open to a deal.

For his own safety, Mr Mdlalose will remain in a private cell, said his defense lawyer Dumisani Mabunda. The investigation could take years, he warned, adding that his client was willing to cooperate with police.