A police operation is under way at the home of a British man shot dead in the French Alps along with his wife and two others.

Surrey Police, accompanied by French officers, have begun examining the home of Iraqi-born Briton Saad Al-Hilli, 50, in Claygate, 30km south of London.

Al Jazeera's Tim Friend, reporting from Claygate, said: "The search here at the family home is intended to be very thorough by the British and French police."

French prosecutor Eric Maillaud said that all those killed were shot twice in the head. 

Speaking at a news conference, Maillaud said the post-mortem examinations were completed on Friday night but he refused to go into any more detail.

Maillaud said that a possible dispute over money between al-Hilli and his brother, Zaid, is one of the lines of inquiry in the investigation.

Al-Hilli's wife, Iqbal, and a woman thought to be his 74-year-old mother-in-law, were killed in the attack.

Two of Al-Hilli's daughters - aged four and seven - who survived the killings, are under police protection in a French hospital.

They were visited by family members accompanied by a British social worker.

The girls are believed to be the only witnesses to Wednesday's killings, carried out near the popular tourist destination of Lake Annecy.

Maillaud also said investigators had gleaned little from a "moving" chat on Friday with the four-year-old, who was being looked after at a psychiatric hospital in Grenoble after spending eight hours hiding under bodies in the car.

The seven-year-old was badly beaten and was due to undergo a second operation for severe head injuries at Grenoble University Hospital.

A French cyclist was also found shot dead at the scene on a mountain road near the village of Chevaline, close to the Annecy Lake and the Swiss border.

The man, a young father who lived in the area named Sylvain Mollier, "just happened to be riding by" at the time of the attack, officials said.

French police are looking into reports of a green or dark-coloured four-wheel drive vehicle and a motorcycle, apparently seen by the cyclist who discovered the murder scene.