French police were using two sonar units late Sunday to scan a river bed in southwest France for the bodies of a Chinese tycoon and a French winemaker missing after their helicopter crashed.

Efforts to find the remains of Lam Kok, a 46-year-old Chinese tea-and-property magnate, his interpreter and financial advisor, Peng Wang, and James Gregoire, a French entrepreneur and the pilot of the helicopter, have been ongoing since Friday, when the aircraft fell into the Dordogne river.

The crash occurred while they had been on a celebratory aerial tour of a chateau estate that the Chinese businessman had just bought from Gregoire.

Police early Saturday located the mangled wreckage of the yellow-and-black four-seater helicopter in seven metres (23 feet) of water in the river and recovered the body of Lam Kok's 12-year-old son, Shun Yu Kok, from its rear.

The bodies of the three other occupants were not found, however, and were believed washed away by strong currents.

The two specialised sonar units were being zig-zagged across the river by boats through the night while more than 100 police using sniffer dogs scoured the river banks.

Police helicopters had also been used during daylight to fly over an expanded zone upstream and downstream.

Crashed helicopter to be hauled out of river

Authorities were to haul the crashed Robinson R44 helicopter out of the river on Monday.

"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 France's air transport investigation department.

The Chinese tycoon's wife had been meant to be on the doomed flight, but declined at the last minute, explaining she was afraid of helicopters.

She and her husband headed a Hong Kong company, Brilliant, that dealt in upmarket teas, luxury hotels and the construction and management of shopping centres.

On Thursday, they had bought the Chateau de La Riviere and its 65-hectare (160-acre) of vineyards for a reported 30 million euros ($41 million) with the aim of turning it into an elite tea- and wine-tasting retreat and plans to build a hotel nearby.

After a lavish event Friday publicising their purchase, Gregoire, who owned two other wine properties in the region, offered to take them up quickly in his helicopter to survey their new purchase.

When the helicopter did not return after 20 minutes, employees at the vineyard contacted emergency services.

Gregoire himself had bought the property in 2003 -- a year after the previous owner died in a plane crash.

A Buddhist ceremony planned by the Chinese couple to promote prosperity at the chateau would go ahead as scheduled on Tuesday, the estate said in a statement to AFP.

The Brilliant group also issued a statement saying it intended to remain involved in the chateau despite the tragedy.

Wealthy Chinese have developed a taste for fine French wines, and their buying power has been credited with pushing prices for certain vintages to record levels.

In recent years they have increasingly taken to buying French vineyards as well.