The elder son of retired senior leader Zhou Yongkang is under formal detention and the family has sought lawyers to prepare his defence, according to sources with direct knowledge of the matter.
Zhou Bin, 41, is seeking legal assistance over possible bribery and other charges, according to three people familiar with the case who declined to be named.
Zhou Bin is allegedly involved in illegal dealings in Sichuan and the oil industry.
He has been formally detained since December.
His detention is a crucial step towards launching judicial proceedings against Zhou, and possibly also his father, who would become the highest-ranking party leader in decades to be embroiled in a graft scandal.
Zhou Yongkang is the first sitting or retired Politburo standing committee member to be investigated for economic or social crimes since the end of the Cultural Revolution. He was formerly in charge of the massive domestic security apparatus.
The decision to investigate him was agreed by current and retired senior leaders in early August.
"Zhou Bin has contacted several lawyers through an intermediary but the final appointment has yet to be made," one of the sources said.
"It's a complicated case and the lawyers must take many considerations into account."
The person added that the lawyers who had been contacted had not seen Zhou Bin in person, due to his detention.
Normally, the appointment of attorneys in a politically sensitive case must be approved and finalised by the top leadership.
Another person said the fact that Zhou Bin had been granted access to lawyers meant he was likely to be brought before a court, and his trial could begin soon.
"The top leaders want the cases surrounding Zhou Yongkang to strictly follow judicial procedures, to send the message that China is governed by rule of law and that the investigation is not a political purge," the source added.
The team investigating Zhou, which reports, directly to President Xi Jinping , recruited another 10 high-level Beijing police officers last month to speed up the interrogations, a person familiar with the operation said.
At least two people told the South China Morning Post Zhou Yongkang was under formal detention in December, and that provincial party secretaries had been informed of the decision.