A confidential autopsy report says that Mpumalanga politician James Nkambule, who died suddenly last week, was poisoned.
Nkambule, 37, was the whistle-blower who earlier this year claimed that politicians were behind assassinations in the province.
He alleged that corruption surrounding the building of the province's multibillion-rand Mbombela stadium for the World Cup led to the deaths.
Nkambule collapsed and died at his home in Mjindini on Thursday night last week.
At the time he was attempting to get a Mozambican man - who he believed to be the hit man - placed in the witness protection programme.
The autopsy, conducted by Mpumalanga chief medical officer Dr Gantcho Gantchev, concludes his death was "unnatural".
In the postmortem report, Gantchev describes "white foamy material" found in Nkambule's throat and windpipe, and about 30ml of brownish fluid "suggestive of ... poison ingestion" in his stomach.
Gantchev told the Sunday Times: "There is no smoke without fire."
"They killed him, they killed my dad," Nkambule's daughter, Buhle Nkambule, said on hearing the news.
Toxicology tests will establish the type of poison and when it entered his system.
Nkambule first rose to prominence in 2001 when he claimed there was a plot to overthrow then president Thabo Mbeki.
He had been branded a "professional liar" by the ANC after publicly accusing members Mathews Phosa, Tokyo Sexwale and Cyril Ramaphosa of being behind the move.
The ex-ANC member also recently claimed that Mpumalanga premier David Mabuza paid R400000 towards President Jacob Zuma's wedding last year. These claims were never denied by the Presidency.
His wife, Claudia Xwabe, said this week that Dr Gantchev had told the family that the postmortem showed there was "a drug in (Nkambule's) body".
On Friday, police confirmed that "foul play" was suspected.
"We have opened an inquest into the death," said provincial police spokesman Captain Leonard Hlati.
Nkambule had recently met with police commissioner General Bheki Cele to discuss efforts to bring a Mozambican man, known in the criminal underworld as "Josh", to South Africa, where he was to be placed under a witness protection programme.
Josh was to testify against prominent figures regarding the death of former Mbombela municipality speaker Jimmy Mohlala. Mohlala was killed after he blew the whistle on irregularities in the awarding of tenders to build the R2-billion Mbombela stadium.
Mohlala's name was on a hit list of nine municipal officials opposed to awarding the contract for the construction of the stadium.
Josh has since claimed in an affidavit, now in the possession of the police, that he was hired by Mpumalanga officials and a soccer boss as a "cleaner" from 2000 until 2009 to eliminate political and business opponents.
His work included smuggling drugs, poisoning people and carrying out other "hits".
He said that the people he targeted included Nkambule; a former mayor of the Gert Sibande district council, Andries Gamede; Scopa chairman Fish Mahlalela; and Mbombela mayor Lassy Chiwayo.
He further alleged that Govan Mbeki municipality chief financial officer Joshua Ntshuhle's car was driven to Malawi to make it look like he had vanished. Ntshuhle went missing in December 2005, days before he was due to testify in the fraud and corruption trial of the municipality's marketing manager, Sibusiso Sigudla. The gangster further claimed that he was hired, along with three Zimbabwean nationals and a fellow Mozambican, to kill Mohlala.
He said four of the men who helped carry out the hit on Mohlala had since been killed.
Nkambule recently slammed the police investigations into the murders - which might have also exposed the tender irregularities - as slow.
He was due to appear in court this week on charges of fraud and defeating the ends of justice after police accused him of fabricating the sworn statement by Josh.
The Sunday Times has established that the police have yet to question anyone in connection with Nkambule's earlier complaint that he had been poisoned. The politician had maintained he was poisoned in 2006 by political opponents, leaving him having to be hospitalised several times.
In August last year, Nkambule said, Josh approached him and confessed that he had poisoned him on instructions from a senior politician.
In a statement seen by the Sunday Times, Josh details how he was handed "a small bottle that was full of liquid substances" in 2006, and instructed to infiltrate a company catering for a matric dance in Barberton where Nkambule was a guest speaker.
"I did in September 2006 ... empty the bottle ... in James Nkambule's food and I went to personally serve him at his table," he said in his statement.
Josh claimed he was prompted to confess to Nkambule after he learnt that his fellow assassins had all been executed to destroy evidence.
Nkambule submitted the statement to police in February - whereupon he was arrested on suspicion of fabricating the statement.
Hlati this week rubbished Nkambule and Josh's claims: "We did not question any person because we did not receive any statement from Josh," he said on Friday. "If you have this Josh, please produce him."
- Read the postmortem report (Pdf format)