Thamsanqa Jantjie said in a 45-minute interview with The Associated Press that his hallucinations began while he was interpreting and that he tried not to panic because there were "armed policemen around". He added that he was once hospitalised in a mental health facility for more than one year.
A South African deputy Cabinet minister, Hendrietta Bogopane-Zulu, later held a news conference to announce that "a mistake happened" in the hiring of Jantjie.
Government officials have tried to track down the company that provided Thamsanqa Jantjie but the owners "have vanished into thin air", said deputy minister of women, children and people with disabilities Hendrietta Bogopane-Zulu.
In this picture taken on December 10, 2013 US President Barack Obama delivers a speech next to a sign language interpreter (R) during the memorial service for late South African President Nelson Mandela at Soccer City Stadium in Johannesburg. AFP photo
She apologised to deaf people offended around the world for Jantjie's incomprehensible signing, and said an investigation is under way to determine how Jantjie was hired and what vetting process, if any, he underwent for his security clearance.
The deputy minister said the translation company offered sub-standard services, the rate they paid the translator was far below the normal levels and that in order to maintain the interpreter's concentration level, interpreters must be switched every 20 minutes. Jantjie was on the stage for the entire service that lasted more than three hours.
She declined to say who in South Africa's government was responsible for contracting the company that provided the translator, or how those rules could be flouted.
"It's an interdepartmental responsibility," she said. "We are trying to establish what happened."
Jantjie, who stood gesticulating three-feet (1 meter) from Obama and others who spoke at Tuesday's ceremony that was broadcast around the world, insisted in the AP interview that he was doing proper sign-language interpretation of the speeches of world leaders.
But he also apologised for his performance that has been dismissed by many sign-language experts as gibberish.
"I would like to tell everybody that if I've offended anyone, please, forgive me," Jantjie said. "But what I was doing, I was doing what I believe is my calling, I was doing what I believe makes a difference."
The statements by Jantjie raise serious security issues for Obama, other heads of state and U.N. Secretary-General Ban Ki-moon who made speeches at FNB Stadium in Soweto, Johannesburg's black township. The ceremony honored Mandela, the anti-apartheid icon and former president who died on December 5. Many of them, including Obama, stood one yard (meter) away from Jantjie.
"What happened that day, I see angels come to the stadium ... I start realizing that the problem is here. And the problem, I don't know the attack of this problem, how will it comes. Sometimes I react violent on that place. Sometimes I will see things that chase me," Jantjie said.
"I was in a very difficult position," he added. "And remember those people, the president and everyone, they were armed, there was armed police around me. If I start panicking I'll start being a problem. I have to deal with this in a manner so that I mustn't embarrass my country."
Asked how often he had become violent, he said "a lot" while declining to provide details.
Jantjie said he was due on the day of the ceremony to get a regular six-month mental health checkup to determine whether the medication he takes was working, whether it needed to be changed or whether he needed to be kept at a mental health facility for treatment.
He said he did not tell the company that contracted him for the event for about $85 that he was due for the checkup, but said the owner of SA Interpreters in Johannesburg was aware of his condition.
AP journalists who visited the address of the company that Jantjie provided found a different company there, whose managers said they knew nothing about SA Interpreters. A woman answered the phone at a number that Jantjie provided and said it was not for the company, and another phone number went to a voicemail that did not identify the person or company with the number.
Jantjie said he received one year of sign language interpretation at a school in Cape Town. He said he has previously interpreted at many events without anyone complaining.
The AP showed Jantjie video footage of him interpreting on stage at the Mandela memorial service.
"I don't remember any of this at all," he said.
The controversy has overshadowed South Africa's 10-day farewell to Mandela, whose remains were lying in state for a second day on Thursday at the Union Buildings in Pretoria, where he was sworn in as the nation's first black president in 1994.
Revelations about Jantjie's unconventional gestures - experts said he did not know even basic signs such as 'thank you' or 'Mandela' - sparked a hunt for the mystery mimer on Wednesday.
The government, which was in charge of the mass memorial, said it had no idea who he was, as did the ruling African National Congress (ANC), even though footage from two large ANC events last year showed him signing on stage next to Zuma.
Jantjie said he worked for a company called SA Interpreters which had been hired by the ANC for Tuesday's ceremony at Johannesburg's 95,000-seat Soccer City stadium.
"Absolutely. That's what happened," he told the radio. The death of Nobel peace laureate Mandela triggered an outpouring of grief and emotion - as well as celebration and thanksgiving - among his 53 million countrymen and millions more around the world.
His body will lay in state for a third day on Friday before being flown to the Eastern Cape, where it will be buried on Sunday at his ancestral home in Qunu, 700 km (450 miles) south of Johannesburg.