How much wealth had Zhou Yongkang's family amassed before the former security tsar's fall?

An unprecedented graft investigation into former top Communist Party official Zhou Yongkang has triggered wild speculation about the massive wealth allegedly amassed by him.

But it is likely that only a small amount of the alleged wealth, if any, can be linked directly to Zhou. As head of the family, Zhou did not get directly involved in business deals. His protection was layers of insulation between himself and the operations.

At least 37 companies, spreading as far as North America, are either owned by Zhou's family or connected to it, according to corporate documents studied by the South China Morning Post. The businesses include oil production, property development, hydropower, tourism and many more. Reuters earlier reported that the total amount of assets held by the family could be as much as 90 billion yuan, although others expressed doubts over the figure.

"If it is really the case, it's horrifying. Scholars have long been saying 'grey income' and money from corruption together account for more than 30 per cent of China's GDP, but that's too much," said Hu Xingdou , a political commentator at the Beijing Institute of Technology.

The name "Zhou Yongkang", however, does not appear in the thousands of pages of corporate documents the Post has studied.

His elder son Zhou Bin, 42, is the one who holds strings of the business empire. Zhan Minli - Zhou Bin's mother-in-law - is the other key player. Zhou's nephew Zhou Feng and his sister-in-law Zhou Lingying also played a part.

Together, these family members hold this vast business empire, either directly or indirectly through people who owed political careers or business fortunes to the senior Zhou.