The sign language interpreter at Nelson Mandela's memorial says he suffers from schizophrenia and hallucinated and saw angels while gesturing incoherently close to President Barack Obama and other world leaders, outraging deaf people worldwide who said his signs amounted to gibberish.

South African officials scrambled on Thursday to explain how they came to hire the man and said they were investigating what vetting process, if any, he underwent for his security clearance.

"In the process, and in the speed of the event, a mistake happened," deputy Cabinet minister Hendrietta Bogopane-Zulu said.

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She apologised to deaf people around the world who were offended by the incomprehensible signing.

The man at the centre of the controversy said in an interview with The Associated Press on Thursday that he began hallucinating while onstage in the stadium filled with tens of thousands of people and that he tried not to panic because there were "armed policemen around me".

Thamsanqa Jantjie added that he has schizophrenia, was once hospitalised in a mental health facility for 19 months and has been violent in the past.

The disclosures raised serious security concerns for Obama, UN Secretary-General Ban Ki-moon and other dignitaries who stood next to Jantjie as they eulogised Mandela at FNB Stadium in Soweto, the black township at the centre of the struggle against racist white rule.

Mandela died on December 5 at 95.

Jantjie has been seen on video performing sign language interpretation at other prominent events in South Africa criticised as fake by advocates for the deaf, including at an appearance last December with South African President Jacob Zuma.

Source: Associated Press