A Frenchman connected to disgraced Chinese politician Bo Xilai has returned to China to help the investigation into the country's biggest political scandal in decades, Cambodian police said Wednesday.

Architect Patrick Devillers, who was arrested by Cambodia last month at Beijing's request, was freed on Monday and boarded a flight for Shanghai the following day "by his own will," deputy national police chief Sok Phal told AFP.

"He said he is going there to be a witness," the official said. "The French embassy supported this 100 percent."

Devillers, 52, was detained in the Cambodian capital on June 13 for unspecified offences but was not formally charged.

Phnom Penh said last week that Beijing had promised to grant Devillers legal immunity in return for cooperating with its investigations.

The Frenchman is understood to have been a close business associate and friend of Bo and his wife Gu Kailai.

The French embassy in Phnom Penh did not immediately respond to requests for comment.

Bo, the former leader of the southwestern Chinese megacity of Chongqing, is being probed for corruption while Gu has been detained for suspected involvement in the murder of British businessman Neil Heywood last year.

The scandal surrounding Bo and Gu, which first came to light in February and made worldwide headlines, has exposed deep divisions within the Communist Party ahead of a crucial, once-in-a-decade leadership transition, analysts say.