The signer, Thamsanqa Jantjie, had stood just feet from the United States president and other world leaders on the stage at last Tuesday's event at Soweto's World Cup stadium.
Enraged sign language experts said the signing by Jantjie, who claims to be schizophrenic, amounted to little more than "flapping his arms around".
Raising further security questions, the Sunday Times newspaper reported that Jantjie had admitted being part of a mob that burnt two people to death in 2003.
Contacted by AFP today, Jantjie dismissed the report as "nonsense".
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"I don't know that. Sunday Times is talking nonsense" he said, but refused to elaborate.
Jantjie directed AFP to the national prosecuting authority to verify if he had any criminal record.
But the prosecution agency has already declined to "confirm or deny" the allegations, and said it does not keep records of closed cases.
The private TV channel eNCA reported Friday that the signer had also faced rape, kidnapping and theft charges.
The African National Congress (ANC), South Africa's ruling party, said it was probing claims that some of its members were directors of the company for which the signer worked.
The Sunday Times said the head of the ANC's religious affairs department owns the company, South African Interpreters.
"We shall also seek to uncover the veracity of these allegations," the party said in a statement on Monday, vowing to "act decisively where any wrong doing against anyone in the employ of the ANC is proven".