The family of dead businessman Sandile Majali appointed an independent pathologist to examine his body, his lawyer John Ngcebetsha said on Monday.
The 48-year-old Majali was found dead in his room at the Sandton Quatermain Hotel in Johannesburg on Sunday morning.
Police said on Monday that they were waiting for tests to be done on medication found in the room.
"It's still too early to talk about the tests. We are waiting for the department of health to give us its findings which will inform the direction the investigation will take," said Gauteng police spokesperson Lieutenant Colonel Lungelo Dlamini.
Ngcebetsha said Majali's death was "highly unlikely" to have been a suicide.
"There is no evidence that suggests that at all. It seems highly unlikely at this point," Ngcebetsha told Sapa.
Ngcebetsha said the family, who was "extremely concerned", confirmed that at the time of his death Majali had been suffering from a "respiratory challenge" for which he was undergoing treatment.
He had been staying at the hotel to rest and receive treatment at the time of his death.
Media asked to back off
Ngcebetsha also said that Majali's family believed that there had been attempts to unfairly persecute him during his life.
"They call for the media to back off from this and let him rest peacefully."
The family still believed in his innocence and saw him as a "trustworthy and honest" person.
They were proud of his achievements in terms of business, entrepreneurship and the economic transformation of South Africa, said Ngcebetsha.
He said the family wanted to protect Majali's assets for his children.
"There are a number of vultures who want things differently."
Majali, who was said to be in financial trouble, came into the public eye following his role in the so-called Oilgate saga that saw him "donate" R11m of PetroSA's funds to the ANC ahead of the 2004 elections.
He was arrested in October 2010 by the SAPS commercial crimes unit on fraud charges after directorship of mining company Kalahari Resources were changed from Brian Amos Mashile and his sister Daphne Mashile-Nkosi to a group of eight people, including Majali.
Majali, who was granted bail, was due to appear in the Johannesburg Specialised Commercial Crimes Court again on January 18.
Oil-for-food
Meanwhile, the AFP news agency reports that Majali was also accused of paying kickbacks to Saddam Hussein's regime during Iraq's graft-plagued oil-for-food programme.
A 2005 UN report on corruption in the programme, which allowed sanction-starved Iraq to sell its crude in return for food and medicine, said Majali had agreed to pay kickbacks to the Saddam regime in return for oil contracts.
Majali was never prosecuted for his alleged role in the oil-for-food scandal.
According to media reports on the leaked findings of a presidential commission appointed to investigate the charges, Majali could not be tried locally because South African law does not cover violations of UN sanctions.
The UN report on the scandal found that Saddam's government ultimately received $228.8m in illicit income from the payment of illegal "surcharges" by Majali and others.