French police have stepped up their search for a Chinese billionaire and two others feared dead after their helicopter crashed as he was surveying a newly purchased vineyard in southwest France.
There was little hope of finding alive Lam Kok, a 46-year-old Chinese tea and property tycoon, his interpreter and adviser Peng Wang, or winemaker James Gregoire after police on Saturday recovered the body of Lam's 12-year-old son from the back of the wrecked helicopter lying in seven metres of water in the Dordogne river.
The four had boarded the helicopter piloted by Gregoire on Friday to make a celebratory tour of a Bordeaux chateau and wine-growing estate that Lam had just bought from the Frenchman.
After pausing the search on Saturday night, divers resumed scouring the cold waters of the Dordogne around 9.00am (1900 AEDT) on Sunday, working in near total obscurity.
"We are meticulously combing the area, with practically no visibility, almost by hand," local gendarmerie commander Ghislain Rety said.
Police said strong currents were hampering the search.
Helicopters with infrared cameras were flying over the river and a special sonar designed for finding bodies was deployed after arriving from the French city of Strasbourg, Rety said.
Slowly dragged behind a boat, the sonar criss-crossed the river between its banks, where about 100 police were also carrying out searches.
An autopsy was to be conducted on Sunday on the body of Lam's son, Shun Yu Kok, but officials said there was little hope the reasons for the crash would be quickly identified.
"The causes of the accident have not been determined at this stage and most likely will not be for several weeks," local prosecutor Christophe Auger said.
"Every lead is being followed - the weather, the rules, maintenance, the pilot's qualifications and the characteristics of the flight," said Philippe Mole of the police air transport investigation department.
The body of the yellow-and-black Robinson R44 helicopter was to be lifted from the water on Monday.
Lam and his wife headed a Hong Kong-based group named Brilliant, which specialises in rare teas and luxury hotels in China.
Newspaper Le Journal du Dimanche reported that Lam had paid more than 30 million euros ($A46.59 million) ($41 million) for the chateau and vineyards.
