British Scenes of Crime forensic officer inside the home of Saad al-Hilli, in Claygate, England, as investigations continue into the death of four people shot dead in the French Alps.

British police on Monday sent a bomb disposal squad to the house of a family killed in the French Alps after finding possible explosives in the latest dramatic twist in the case.

Police also evacuated neighbouring homes after the discovery at the home of Iraqi-born Saad al-Hilli, who was shot dead with his wife and mother-in-law in their car at a beauty spot in eastern France last Wednesday.

The find will add to exotic theories about the execution-style killings, over which police are still quizzing Hilli's brother, who has denied reports that a property dispute was at the root of the killings.

The local Surrey police force said they had evacuated the immediate area around the Hilli family home in the village of Claygate, south-west of London, due to “concerns around items found at the address”.

“An assessment of items found at the address is currently being carried out as a precaution,” a police spokeswoman told AFP.

A British army Royal Logistics Corps bomb disposal van was present at the scene, an AFP photographer said.

A source close to the inquiry said that a “potentially explosive substance” had been found at the home but did not say what it was.

In France, the elder of Hilli's two daughters, both of whom survived the attack, remained under sedation Monday and was unable to speak to investigators who hope that she can help them unravel the mystery.

Zainab al-Hilli, seven, is recovering from a fractured skull and a bullet wound in the shoulder. Her four-year-old sister Zeena survived the attack unscathed after hiding in the back of the family car.

“When the doctors give us authorisation we will be able to interview her in hospital but for the moment they are not allowing it,” Annecy prosecutor Eric Maillaud told AFP.

“She was in an induced coma which she was brought out of on Sunday but she remains under sedation. It is a normal process.”

Zainab, who lay motionless under her mother's corpse for eight hours following Wednesday's shooting, has also been unable to provide any significant information about the attack.

She returned to Britain on Sunday.

Saad al-Hilli, his wife Ikbal and Ikbal's mother, a Swedish national, were all killed by two shots to the head in the car on a forested Alpine road. A passing French cyclist was also killed.

Hilli, a 50-year-old naturalised Briton of Iraqi origin, worked as a mechanical design engineer with a firm based in Surrey that makes commercial space satellites.

Police were on Monday questioning his brother Zaid al-Hilli for a third day, a source told AFP on condition of anonymity. It is understood he was not held overnight in police custody.

He presented himself to police in Britain following the murder, denying media reports that the brothers were involved in a financial dispute over their father's will.

On Monday morning police suddenly moved back reporters and widened the cordon around the house after armed officers arrived at the mock-Tudor-fronted house.

The army bomb squad arrived later, while forensics officers could also be seen searching the driveways of neighbouring homes.

Five French investigators are in Britain to work on the case.

The attack on the family's car took place outside the village of Chevaline, near the lakeside resort of Annecy in southeast France where the Hillis had been on a camping holiday.

Autopsies revealed that each of the four dead victims were hit by several bullets and shot twice in the head.

French police say they are not solely focusing on the theory that it was an “ordered execution”. - AFP