The men, including one of Jantjie's cousins, insisted on anonymity because of sensitivity surrounding the bogus signing, which has humiliated South Africa's government.
They say Jantjie was institutionalized and then returned to his neighborhood on the outskirts of Soweto.
The accusations come days after it was claimed he has faced charges for murder, rape and kidnapping.
South African news website eNCA reported that Mr Jantjie, who has schizophrenia, has faced charges for rape (1994), theft (1995), housebreaking (1997), malicious damage to property (1998), murder, attempted murder and kidnapping (2003) charges.
'Necklacing' is the practice of forcing a tyre filled with petrol over victim's head and shoulders and setting it alight.
It can often take a victim more than 20 minutes to die in excruciating agony.
In the violent 1980s and 1990s, necklacing was a common sentence imposed by 'people's courts' on collaborators with the apartheid regime and criminals in South Africa.
It was frequently carried out in the name of the African National Congress and was alleged to have been endorsed by Nelson Mandela's then wife, Winnie. The ANC says it never condoned necklacing.
In 1986 Mrs Mandela, caused controversy when she stated: 'With our boxes of matches and our necklaces we shall liberate this country.'
The statement, which was widely seen as an implicit endorsement of necklacing, caused the ANC to briefly try to distance itself from her.
It is still used in certain, more lawless, parts of Africa, where corrupt police are no longer trusted, to punish thieves and rapists.
Incidents have been reported more recently in Haiti, Ivory Coast, Nigeria and India.
Brazilian drug lords are also known to have 'necklaced' their enemies, most notoriously the journalist Tim Lopes in 2002.
The website said it was unclear if the 2003 murder case was ever concluded as the court file was found to be empty during their investigations.
It also reported that many of the charges brought against him were dropped, allegedly because he was mentally unfit to stand trial.
Mr Jantjie was acquitted on the rape charge, but he was convicted of theft for which he was sentenced to three years in prison. The channel could not ascertain if he served the jail time.
MailOnline has contacted the NPA for a comment on the claims.
Mr Jantjie was yesterday approached by a reporter for the Associated Press who asked him about the criminal charges, but he refused to comment.
The news is a further embarrassment to South African officials as it was revealed that Mr Jantjie had faked sign language at the memorial event.
Twitter users with sign language knowledge claimed the interpreter repeatedly used signs for 'donkey', 'lightning bolt' and 'prawns'.
Eye Witness News meanwhile has reported trained sign language trainers as saying he also made reference to 'rocking horses'.
Braam Jordaan, a deaf South African and board member of the World Deaf Federation, has said he believed the interpreter was making up signs as he went along.
Last week, Mr Jantjie explained the embarrassment by claiming he may have suffered a schizophrenic episode on stage and that he saw 'angels' at the event.
He said that his hallucinations began while he was interpreting and that he tried not to panic because there were 'armed policemen around me.'
- Daily Mail
