The four, who wanted to stay anonymous due to the sensitivity of the signing incident, say Thamsanqa Jantjie was in a group that killed two men by placing tires around their necks and then setting them on fire in 2003.
Jantjie never went to trial because authorities determined he was not mentally fit, they said. Instead, he was institutionalized for a period of longer than a year and then returned to live in his poor township neighborhood on the outskirts of Soweto, according to the group.
After Jantjie returned home, he started getting jobs doing sign language interpretation at events for the governing African National Congress Party, the group said.
Last week, Jantjie said he has schizophrenia and hallucinated during the Mandela ceremony. He said he saw angels while gesturing incoherently just 3 feet away from President Barack Obama and other world leaders. Signing experts said his arm and hand movements were gibberish.
The fake signing incident has deeply embarrassed South Africa's government and prompted a high-level investigation into how it happened.
The Associated Press contributed to this report.