South Africa's deputy disability minister says the government is investigating how a man who faked sign language at the Mandela memorial was given security clearance.
Thamsanqa Jantjie, who stood alongside world leaders at the event, has denied being a fraud, and said he panicked when he began hallucinating.
He said he had schizophrenia, which had in the past made him act violently.
The agency that employed him, SA Interpreters, has reportedly vanished.
The African National Congress (ANC) said it had used Mr Jantjie as an interpreter several times before, and "had not been aware of any of complaints regarding the quality of services, qualifications or reported illnesses" of the interpreter.
But it said Tuesday's memorial at a stadium in Johannesburg was organised by the state, not the ANC, so the ruling party could not comment on security arrangements.
The South African Translators' Institute said earlier there had been complaints over Mr Jantjie's work before, but that the ANC had taken no action.
The ANC said it would "follow up the reported correspondence that has supposedly been sent to us in this regard and where necessary act on it".
Mr Mandela died last week at the age of 95, and will be buried on Sunday.
His body is currently lying in state in Pretoria, with thousands queuing to pay their respects.
During the memorial, Mr Jantjie (also spelt Dyantyi) stood on the stage next to key speakers including US President Barack Obama, South African President Jacob Zuma and Mr Mandela's grandchildren, translating their eulogies.
Mr Jantjie's performance was watched on television by millions of people worldwide and angered the South African deaf community. Pressure has been mounting on the government to explain why he was hired for such an important event.
Deputy Disability Minister Hendrietta Bogopane-Zulu told a news conference on Thursday: "Firstly, I don't think South Africa as a country would put at risk anybody's security, especially those of heads of state.
"Secondly, when somebody provides a service of a sign language interpreter, I don't think... somebody would say: 'Is your head ok? Do you have any mental disability?' I think the focus was on: 'Are you able to sign? Can you provide the services?'"
But she said: "In terms of security clearance that is in a process, we are requesting to check his vetting."
Ms Bogopane-Zulu apologised to the deaf community but said there was no reason for the country to be embarrassed.
"There are as many as a hundred sign language dialects," she said, explaining that Mr Jantjie speaks Xhosa and that "the English was a bit too much for him".
She also accused Mr Jantjie's employers of being "cheats", and said the directors of SA Interpreters had since vanished.
Mr Jantjie himself has blamed his flawed interpretation on a schizophrenic episode.
He told the Associated Press he had often been violent in the past, and had been due to attend a routine mental health check-up on the day of the memorial, to determine whether he needed to be admitted to hospital.
He told the BBC that during the event he had had a breakdown, and started hallucinating that angels were coming down into the crowd.
"I started knowing that I am not real, because it's not something possible. But believe me I saw them coming on stage.
"From that moment, it was not myself," he said, saying he had becoming concerned for the safety of people in the stadium and was "absolutely" aware that he was not signing correctly.
The US Secret Service said "agreed upon security measures" had been in place for President Obama's appearance at the memorial, and that US agents were "always in close proximity" to him wherever he went.
Spokesman Brian Leary said it was the responsibility of the South Africans to carry out the relevant background checks on people involved.
"Beyond that we won't comment on deliberations that took place between the Secret Service and South African authorities," he said.
Mr Mandela's body is lying in state until 13 December, when the military will fly the coffin to the Eastern Cape from Air Force Base Waterkloof in Pretoria.
A military guard of honour will welcome the arrival, and the coffin will then be placed on a gun carriage and transported to a hearse.
Mr Mandela's body will then be taken to his home village of Qunu, where the Thembu community will conduct a traditional ceremony.
A national day of reconciliation will take place on 16 December when a statue of Mr Mandela will be unveiled at the Union Buildings.
Big screens have been set up across South Africa to show the planned national events.