The billionaire tycoon who testified that he paid former rising Communist Party political star Bo Xilai millions in bribes has died in prison, Chinese state media reported Monday.

Xu Ming, the chief accuser at the trial of the fallen Chongqing party chief, probably died of a "sudden heart attack", the official Xinhua news service said, despite being only 44.

But social media users asked whether Xu's death -- months before his scheduled release -- could have been more than a coincidence.

State media largely glossed over Xu's central role in one of the country's most lurid political episodes of recent decades. Instead it described him as the founder of conglomerate Shide Group and former chairman of Chinese football champions Dalian Shide, an indication of the case's continued sensitivity.

Bo, whose wife was convicted of killing a British businessman, was accused of accepting almost $3.5 million in bribes from Xu, who also provided the family with funds for a French villa. The politician was jailed for life for graft in 2013.

Xu got his start selling prawns, before founding Shide Group in the early 1990s, building it into a national conglomerate with interests in everything from construction and petrochemicals to sport.

His close ties to Bo's family allegedly helped him to expand his business empire and increase his personal fortune to over a billion dollars. At one time he was listed as the eighth richest person in China.

In 2000 he bought China's then most successful football team and renamed it after his company.

The club was left in financial limbo after his detention and played its last game in 2012 before being acquired by the Aerbin Group.

Xu was due to be released in September 2016 but died on Friday, Xinhua said.

"His honour and shame will both follow his ashes eternally into the grave," it said.

- 'Rebels and traitors' -

Many media outlets across China used almost identical wording to describe the circumstances of Xu's life and death, suggesting that authorities may have issued instructions on how to report the news -- often the case for sensitive subjects.

The offences he was convicted of remained unclear, with state media saying only that he was detained in March 2012 in connection with "economic cases" before being "punished and sent to jail".

Afterwards, "the outside world could get very little news" about his fate, said the Beijing Times.

Despite the official circumspection, Chinese social media were rife with speculation about the case, with many noting that some high officials might have wanted to see Xu dead because he knew too much.

"Some people feared he would expose the truth, so he died! This is proof that the same rebels and traitors still hold power," wrote one user on Sina Weibo, a Chinese version of Twitter. The comment was later censored.

"This will make people more suspicious," said another deleted post. "Who is still behind Xu Ming?"

It would be interesting to learn more about the "sensitive subject of that French villa", said another commentator. "It's a pity that we'll remain in suspense forever".

Although a few outlets made passing reference to Xu's links to Bo, most preferred to focus on his high-profile role in Chinese soccer.

Avoiding any mention of his links to Bo, the state-run China Youth Daily used his death as an excuse to lambast Chinese football for its history of corruption and gambling.

Even in jail, "he continued to have a high degree of interest in football", according to the stock language used by several news outlets, which added that he fell into "despair" over the poor performances of his former team.