Johannesburg (CNN) -- The sign language interpreter widely ridiculed for his performance at the Nelson Mandela memorial stands by his work.
Thamsanqa Jantjie said Thursday that he is a fully qualified interpreter and has been trusted in the past with other big events.
"It has been many years I have been doing this job" he told CNN. "My portfolio shows that I have been a champion of what I have been doing."
Not so, says the head of the South Africa Translators' Institute.
There were complaints last year after Jantjie interpreted the proceedings at the ruling African National Congress elective conference, the institute's chairman, Johan Blaauw, told the South African Press Association.
But Jantjie stands by his work. "I have never in my life had anything that said I have interpreted wrong," he said.
"For the deaf association, if they think that I have done a wrong interpretation, I ask forgiveness. For Deaf SA, if they are telling me that I was doing a wrong interpretation, then they should answer me why they were silent all the time, all these years," he said, referring to the Deaf Federation of South Africa, which has criticized his work at the service.
Jantjie declined to provide details about his hire for the four-hour memorial, watched by millions around the world, as a government inquiry looks into the matter.
He said he had been drawn to the job of interpreting because he was disabled.
"I am suffering from schizophrenia, which is controllable. I am under treatment," Jantjie said.
At a news conference Thursday, Hendrietta Bogopane-Zulu, South Africa's deputy minister of women, children and people with disabilities, admitted that mistakes had happened at the memorial service but added that Jantjie was not a "fake."
She said there was no sign language standard in South Africa and deaf people spoke different dialects.
She added that the government was looking into the vetting of Jantjie's security clearance at the memorial.
"But normally when you do provide a service at a particular level, you will be vetted. And as you could see, he had his accreditation, so he didn't just walk through," she said.
Bogopane-Zulu also said the government tracked down the company Jantjie worked for, but the owners "seemed to have vanished."
"We managed to get hold of them and then we spoke to them, wanting some answers, and they vanished into thin air," she said. "They have been providing substandard service for years."
Jantjie named his employer as a company called SA Interpreters. He did not give details about his training, saying his qualifications are filed with the company.
"You can look at my portfolio, it speaks for itself from the events that I have done in my country," he said.
As outrage over his interpretation skills have grown, so have questions about who hired him.
The ANC said it did not hire Jantjie for the service, as it was the state that had organized the memorial.
"The processes that were followed to procure Mr. Jantjie's services were thus government processes and not ANC processes," the ruling party said in a prepared statement, adding that it had used his services before.
"It is important to make the point that, up until yesterday, the African National Congress had not been aware of any of complaints regarding the quality of services, qualifications or reported illnesses of Mr Jantjie."
Jantjie said he was proud to receive a call saying he would be interpreting at the memorial for the revered statesman, who died last week at age 95.
Asked who had called him to tell him he would be interpreting, he just said: "It is a lady that I am serving under."
He said the atmosphere on the day was "wonderful."
"It was a dream come true," he said, adding that he felt as if he had reached "the standard of the champion of the whole world."
At the service, at which world leaders from President Barack Obama to Cuba's Raul Castro spoke, he said he was doing South African sign language.
But the national director of the Deaf Federation of South Africa sees it differently. He said Jantjie had been dubbed a "fake interpreter."
"The deaf community is in outrage," said Bruno Druchen. "He is not known by the Deaf Community in South Africa nor by the South African Sign Language interpreters working in the field."
The man showed no facial expressions, which are key in South African sign language, and his hand signals were meaningless, Druchen said. "It is a total mockery of the language," he added.
While dignitaries addressed the crowd at Johannesburg's FNB stadium, Jantjie produced a series of hand signals that experts said meant nothing.
"It was almost like he was doing baseball signs," deaf actress Marlee Matlin told CNN on Wednesday, through a sign language interpreter. "I was appalled."
Though each country has its own sign language, all of them entail facial expressions, she said. She called his lack of facial expression "a giveaway."
"I knew exactly right then and there that he wasn't authentic at all, and it was offensive; it was offensive to me."
CNN's Ed Payne and Kim Norgaard contributed to this report.